diff --git a/Makefile b/Makefile
index de53954590..6dfdb33cb2 100644
--- a/Makefile
+++ b/Makefile
@@ -601,7 +601,6 @@ SCRIPT_SH += git-bisect.sh
 SCRIPT_SH += git-difftool--helper.sh
 SCRIPT_SH += git-filter-branch.sh
 SCRIPT_SH += git-merge-octopus.sh
-SCRIPT_SH += git-merge-one-file.sh
 SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
 SCRIPT_SH += git-quiltimport.sh
@@ -909,6 +908,7 @@ LIB_OBJS += match-trees.o
 LIB_OBJS += mem-pool.o
 LIB_OBJS += merge-blobs.o
 LIB_OBJS += merge-recursive.o
+LIB_OBJS += merge-strategies.o
 LIB_OBJS += merge.o
 LIB_OBJS += mergesort.o
 LIB_OBJS += midx.o
@@ -1094,6 +1094,7 @@ BUILTIN_OBJS += builtin/mailsplit.o
 BUILTIN_OBJS += builtin/merge-base.o
 BUILTIN_OBJS += builtin/merge-file.o
 BUILTIN_OBJS += builtin/merge-index.o
+BUILTIN_OBJS += builtin/merge-one-file.o
 BUILTIN_OBJS += builtin/merge-ours.o
 BUILTIN_OBJS += builtin/merge-recursive.o
 BUILTIN_OBJS += builtin/merge-tree.o
diff --git a/builtin.h b/builtin.h
index 53fb290963..4d2cd78856 100644
--- a/builtin.h
+++ b/builtin.h
@@ -178,6 +178,7 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix);
 int cmd_merge_index(int argc, const char **argv, const char *prefix);
 int cmd_merge_ours(int argc, const char **argv, const char *prefix);
 int cmd_merge_file(int argc, const char **argv, const char *prefix);
+int cmd_merge_one_file(int argc, const char **argv, const char *prefix);
 int cmd_merge_recursive(int argc, const char **argv, const char *prefix);
 int cmd_merge_tree(int argc, const char **argv, const char *prefix);
 int cmd_mktag(int argc, const char **argv, const char *prefix);
diff --git a/builtin/merge-one-file.c b/builtin/merge-one-file.c
new file mode 100644
index 0000000000..9c21778e1d
--- /dev/null
+++ b/builtin/merge-one-file.c
@@ -0,0 +1,94 @@
+/*
+ * Builtin "git merge-one-file"
+ *
+ * Copyright (c) 2020 Alban Gruin
+ *
+ * Based on git-merge-one-file.sh, written by Linus Torvalds.
+ *
+ * This is the git per-file merge utility, called with
+ *
+ *   argv[1] - original file object name (or empty)
+ *   argv[2] - file in branch1 object name (or empty)
+ *   argv[3] - file in branch2 object name (or empty)
+ *   argv[4] - pathname in repository
+ *   argv[5] - original file mode (or empty)
+ *   argv[6] - file in branch1 mode (or empty)
+ *   argv[7] - file in branch2 mode (or empty)
+ *
+ * Handle some trivial cases. The _really_ trivial cases have been
+ * handled already by git read-tree, but that one doesn't do any merges
+ * that might change the tree layout.
+ */
+
+#define USE_THE_INDEX_COMPATIBILITY_MACROS
+#include "cache.h"
+#include "builtin.h"
+#include "lockfile.h"
+#include "merge-strategies.h"
+
+static const char builtin_merge_one_file_usage[] =
+	"git merge-one-file <orig blob> <our blob> <their blob> <path> "
+	"<orig mode> <our mode> <their mode>\n\n"
+	"Blob ids and modes should be empty for missing files.";
+
+static int read_mode(const char *name, const char *arg, unsigned int *mode)
+{
+	char *last;
+	int ret = 0;
+
+	*mode = strtol(arg, &last, 8);
+
+	if (*last)
+		ret = error(_("invalid '%s' mode: expected nothing, got '%c'"), name, *last);
+	else if (!(S_ISREG(*mode) || S_ISDIR(*mode) || S_ISLNK(*mode)))
+		ret = error(_("invalid '%s' mode: %o"), name, *mode);
+
+	return ret;
+}
+
+int cmd_merge_one_file(int argc, const char **argv, const char *prefix)
+{
+	struct object_id orig_blob, our_blob, their_blob,
+		*p_orig_blob = NULL, *p_our_blob = NULL, *p_their_blob = NULL;
+	unsigned int orig_mode = 0, our_mode = 0, their_mode = 0, ret = 0;
+	struct lock_file lock = LOCK_INIT;
+
+	if (argc != 8)
+		usage(builtin_merge_one_file_usage);
+
+	if (read_cache() < 0)
+		die("invalid index");
+
+	hold_locked_index(&lock, LOCK_DIE_ON_ERROR);
+
+	if (!get_oid_hex(argv[1], &orig_blob)) {
+		p_orig_blob = &orig_blob;
+		ret = read_mode("orig", argv[5], &orig_mode);
+	} else if (!*argv[1] && *argv[5])
+		ret = error(_("no 'orig' object id given, but a mode was still given."));
+
+	if (!get_oid_hex(argv[2], &our_blob)) {
+		p_our_blob = &our_blob;
+		ret = read_mode("our", argv[6], &our_mode);
+	} else if (!*argv[2] && *argv[6])
+		ret = error(_("no 'our' object id given, but a mode was still given."));
+
+	if (!get_oid_hex(argv[3], &their_blob)) {
+		p_their_blob = &their_blob;
+		ret = read_mode("their", argv[7], &their_mode);
+	} else if (!*argv[3] && *argv[7])
+		ret = error(_("no 'their' object id given, but a mode was still given."));
+
+	if (ret)
+		return ret;
+
+	ret = merge_three_way(the_repository, p_orig_blob, p_our_blob, p_their_blob,
+			      argv[4], orig_mode, our_mode, their_mode);
+
+	if (ret) {
+		rollback_lock_file(&lock);
+		return !!ret;
+	}
+
+	return write_locked_index(&the_index, &lock, COMMIT_LOCK);
+}
diff --git a/git-merge-one-file.sh b/git-merge-one-file.sh
deleted file mode 100755
index f6d9852d2f..0000000000
--- a/git-merge-one-file.sh
+++ /dev/null
@@ -1,167 +0,0 @@
-#!/bin/sh
-#
-# Copyright (c) Linus Torvalds, 2005
-#
-# This is the git per-file merge script, called with
-#
-#   $1 - original file SHA1 (or empty)
-#   $2 - file in branch1 SHA1 (or empty)
-#   $3 - file in branch2 SHA1 (or empty)
-#   $4 - pathname in repository
-#   $5 - original file mode (or empty)
-#   $6 - file in branch1 mode (or empty)
-#   $7 - file in branch2 mode (or empty)
-#
-# Handle some trivial cases.. The _really_ trivial cases have
-# been handled already by git read-tree, but that one doesn't
-# do any merges that might change the tree layout.
-
-USAGE='<orig blob> <our blob> <their blob> <path>'
-USAGE="$USAGE <orig mode> <our mode> <their mode>"
-LONG_USAGE="usage: git merge-one-file $USAGE
-
-Blob ids and modes should be empty for missing files."
-
-SUBDIRECTORY_OK=Yes
-. git-sh-setup
-cd_to_toplevel
-require_work_tree
-
-if test $# != 7
-then
-	echo "$LONG_USAGE"
-	exit 1
-fi
-
-case "${1:-.}${2:-.}${3:-.}" in
-#
-# Deleted in both or deleted in one and unchanged in the other
-#
-"$1.." | "$1.$1" | "$1$1.")
-	if { test -z "$6" && test "$5" != "$7"; } ||
-	   { test -z "$7" && test "$5" != "$6"; }
-	then
-		echo "ERROR: File $4 deleted on one branch but had its" >&2
-		echo "ERROR: permissions changed on the other." >&2
-		exit 1
-	fi
-
-	if test -n "$2"
-	then
-		echo "Removing $4"
-	else
-		# read-tree checked that index matches HEAD already,
-		# so we know we do not have this path tracked.
-		# there may be an unrelated working tree file here,
-		# which we should just leave unmolested.  Make sure
-		# we do not have it in the index, though.
-		exec git update-index --remove -- "$4"
-	fi
-	if test -f "$4"
-	then
-		rm -f -- "$4" &&
-		rmdir -p "$(expr "z$4" : 'z\(.*\)/')" 2>/dev/null || :
-	fi &&
-		exec git update-index --remove -- "$4"
-	;;
-
-#
-# Added in one.
-#
-".$2.")
-	# the other side did not add and we added so there is nothing
-	# to be done, except making the path merged.
-	exec git update-index --add --cacheinfo "$6" "$2" "$4"
-	;;
-"..$3")
-	echo "Adding $4"
-	if test -f "$4"
-	then
-		echo "ERROR: untracked $4 is overwritten by the merge." >&2
-		exit 1
-	fi
-	git update-index --add --cacheinfo "$7" "$3" "$4" &&
-		exec git checkout-index -u -f -- "$4"
-	;;
-
-#
-# Added in both, identically (check for same permissions).
-#
-".$3$2")
-	if test "$6" != "$7"
-	then
-		echo "ERROR: File $4 added identically in both branches," >&2
-		echo "ERROR: but permissions conflict $6->$7." >&2
-		exit 1
-	fi
-	echo "Adding $4"
-	git update-index --add --cacheinfo "$6" "$2" "$4" &&
-		exec git checkout-index -u -f -- "$4"
-	;;
-
-#
-# Modified in both, but differently.
-#
-"$1$2$3" | ".$2$3")
-
-	case ",$6,$7," in
-	*,120000,*)
-		echo "ERROR: $4: Not merging symbolic link changes." >&2
-		exit 1
-		;;
-	*,160000,*)
-		echo "ERROR: $4: Not merging conflicting submodule changes." >&2
-		exit 1
-		;;
-	esac
-
-	src1=$(git unpack-file $2)
-	src2=$(git unpack-file $3)
-	case "$1" in
-	'')
-		echo "Added $4 in both, but differently."
-		orig=$(git unpack-file $(git hash-object /dev/null))
-		;;
-	*)
-		echo "Auto-merging $4"
-		orig=$(git unpack-file $1)
-		;;
-	esac
-
-	git merge-file "$src1" "$orig" "$src2"
-	ret=$?
-	msg=
-	if test $ret != 0 || test -z "$1"
-	then
-		msg='content conflict'
-		ret=1
-	fi
-
-	# Create the working tree file, using "our tree" version from the
-	# index, and then store the result of the merge.
-	git checkout-index -f --stage=2 -- "$4" && cat "$src1" >"$4" || exit 1
-	rm -f -- "$orig" "$src1" "$src2"
-
-	if test "$6" != "$7"
-	then
-		if test -n "$msg"
-		then
-			msg="$msg, "
-		fi
-		msg="${msg}permissions conflict: $5->$6,$7"
-		ret=1
-	fi
-
-	if test $ret != 0
-	then
-		echo "ERROR: $msg in $4" >&2
-		exit 1
-	fi
-	exec git update-index -- "$4"
-	;;
-
-*)
-	echo "ERROR: $4: Not handling case $1 -> $2 -> $3" >&2
-	;;
-esac
-exit 1
diff --git a/git.c b/git.c
index f1e8b56d99..a4d3f98094 100644
--- a/git.c
+++ b/git.c
@@ -540,6 +540,7 @@ static struct cmd_struct commands[] = {
 	{ "merge-file", cmd_merge_file, RUN_SETUP_GENTLY },
 	{ "merge-index", cmd_merge_index, RUN_SETUP | NO_PARSEOPT },
 	{ "merge-ours", cmd_merge_ours, RUN_SETUP | NO_PARSEOPT },
+	{ "merge-one-file", cmd_merge_one_file, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive-ours", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive-theirs", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
diff --git a/merge-strategies.c b/merge-strategies.c
new file mode 100644
index 0000000000..f5fdb15bbf
--- /dev/null
+++ b/merge-strategies.c
@@ -0,0 +1,173 @@
+#include "cache.h"
+#include "dir.h"
+#include "merge-strategies.h"
+#include "xdiff-interface.h"
+
+static int checkout_from_index(struct index_state *istate, const char *path,
+			       struct cache_entry *ce)
+{
+	struct checkout state = CHECKOUT_INIT;
+
+	state.istate = istate;
+	state.force = 1;
+	state.base_dir = "";
+	state.base_dir_len = 0;
+
+	if (checkout_entry(ce, &state, NULL, NULL) < 0)
+		return error(_("%s: cannot checkout file"), path);
+	return 0;
+}
+
+static int merge_one_file_deleted(struct index_state *istate,
+				  const struct object_id *our_blob,
+				  const struct object_id *their_blob, const char *path,
+				  unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode)
+{
+	if ((our_blob && orig_mode != our_mode) ||
+	    (their_blob && orig_mode != their_mode))
+		return error(_("File %s deleted on one branch but had its "
+			       "permissions changed on the other."), path);
+
+	if (our_blob) {
+		printf(_("Removing %s\n"), path);
+
+		if (file_exists(path))
+			remove_path(path);
+	}
+
+	if (remove_file_from_index(istate, path))
+		return error("%s: cannot remove from the index", path);
+	return 0;
+}
+
+static int do_merge_one_file(struct index_state *istate,
+			     const struct object_id *orig_blob,
+			     const struct object_id *our_blob,
+			     const struct object_id *their_blob, const char *path,
+			     unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode)
+{
+	int ret, i, dest;
+	ssize_t written;
+	mmbuffer_t result = {NULL, 0};
+	mmfile_t mmfs[3];
+	xmparam_t xmp = {{0}};
+
+	if (our_mode == S_IFLNK || their_mode == S_IFLNK)
+		return error(_("%s: Not merging symbolic link changes."), path);
+	else if (our_mode == S_IFGITLINK || their_mode == S_IFGITLINK)
+		return error(_("%s: Not merging conflicting submodule changes."), path);
+	else if (our_mode != their_mode)
+		return error(_("permission conflict: %o->%o,%o in %s"),
+			     orig_mode, our_mode, their_mode, path);
+
+	if (orig_blob) {
+		printf(_("Auto-merging %s\n"), path);
+		read_mmblob(mmfs + 0, orig_blob);
+	} else {
+		printf(_("Added %s in both, but differently.\n"), path);
+		read_mmblob(mmfs + 0, &null_oid);
+	}
+
+	read_mmblob(mmfs + 1, our_blob);
+	read_mmblob(mmfs + 2, their_blob);
+
+	xmp.level = XDL_MERGE_ZEALOUS_ALNUM;
+	xmp.style = 0;
+	xmp.favor = 0;
+
+	ret = xdl_merge(mmfs + 0, mmfs + 1, mmfs + 2, &xmp, &result);
+
+	for (i = 0; i < 3; i++)
+		free(mmfs[i].ptr);
+
+	if (ret < 0) {
+		free(result.ptr);
+		return error(_("Failed to execute internal merge"));
+	} else if (ret > 0 || !orig_blob) {
+		free(result.ptr);
+		return error(_("content conflict in %s"), path);
+	}
+
+	unlink(path);
+	if ((dest = open(path, O_WRONLY | O_CREAT, our_mode)) < 0) {
+		free(result.ptr);
+		return error_errno(_("failed to open file '%s'"), path);
+	}
+
+	written = write_in_full(dest, result.ptr, result.size);
+	close(dest);
+
+	free(result.ptr);
+
+	if (written < 0)
+		return error_errno(_("failed to write to '%s'"), path);
+
+	return add_file_to_index(istate, path, 0);
+}
+
+int merge_three_way(struct repository *r,
+		    const struct object_id *orig_blob,
+		    const struct object_id *our_blob,
+		    const struct object_id *their_blob, const char *path,
+		    unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode)
+{
+	if (orig_blob &&
+	    ((!their_blob && our_blob && oideq(orig_blob, our_blob)) ||
+	     (!our_blob && their_blob && oideq(orig_blob, their_blob)))) {
+		/* Deleted in both or deleted in one and unchanged in the other. */
+		return merge_one_file_deleted(r->index, our_blob, their_blob, path,
+					      orig_mode, our_mode, their_mode);
+	} else if (!orig_blob && our_blob && !their_blob) {
+		/*
+		 * Added in one.  The other side did not add and we
+		 * added so there is nothing to be done, except making
+		 * the path merged.
+		 */
+		return add_to_index_cacheinfo(r->index, our_mode, our_blob, path, 0, 1, 1, NULL);
+	} else if (!orig_blob && !our_blob && their_blob) {
+		struct cache_entry *ce;
+		printf(_("Adding %s\n"), path);
+
+		if (file_exists(path))
+			return error(_("untracked %s is overwritten by the merge."), path);
+
+		if (add_to_index_cacheinfo(r->index, their_mode, their_blob, path, 0, 1, 1, &ce))
+			return -1;
+		return checkout_from_index(r->index, path, ce);
+	} else if (!orig_blob && our_blob && their_blob &&
+		   oideq(our_blob, their_blob)) {
+		struct cache_entry *ce;
+
+		/* Added in both, identically (check for same permissions). */
+		if (our_mode != their_mode)
+			return error(_("File %s added identically in both branches, "
+				       "but permissions conflict %o->%o."),
+				     path, our_mode, their_mode);
+
+		printf(_("Adding %s\n"), path);
+
+		if (add_to_index_cacheinfo(r->index, our_mode, our_blob, path, 0, 1, 1, &ce))
+			return -1;
+		return checkout_from_index(r->index, path, ce);
+	} else if (our_blob && their_blob) {
+		/* Modified in both, but differently. */
+		return do_merge_one_file(r->index,
+					 orig_blob, our_blob, their_blob, path,
+					 orig_mode, our_mode, their_mode);
+	} else {
+		char orig_hex[GIT_MAX_HEXSZ] = {0}, our_hex[GIT_MAX_HEXSZ] = {0},
+			their_hex[GIT_MAX_HEXSZ] = {0};
+
+		if (orig_blob)
+			oid_to_hex_r(orig_hex, orig_blob);
+		if (our_blob)
+			oid_to_hex_r(our_hex, our_blob);
+		if (their_blob)
+			oid_to_hex_r(their_hex, their_blob);
+
+		return error(_("%s: Not handling case %s -> %s -> %s"),
+			     path, orig_hex, our_hex, their_hex);
+	}
+
+	return 0;
+}
diff --git a/merge-strategies.h b/merge-strategies.h
new file mode 100644
index 0000000000..e624c4f27c
--- /dev/null
+++ b/merge-strategies.h
@@ -0,0 +1,12 @@
+#ifndef MERGE_STRATEGIES_H
+#define MERGE_STRATEGIES_H
+
+#include "object.h"
+
+int merge_three_way(struct repository *r,
+		    const struct object_id *orig_blob,
+		    const struct object_id *our_blob,
+		    const struct object_id *their_blob, const char *path,
+		    unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode);
+
+#endif /* MERGE_STRATEGIES_H */
diff --git a/t/t6415-merge-dir-to-symlink.sh b/t/t6415-merge-dir-to-symlink.sh
index 2eddcc7664..5fb74e39a0 100755
--- a/t/t6415-merge-dir-to-symlink.sh
+++ b/t/t6415-merge-dir-to-symlink.sh
@@ -94,7 +94,7 @@ test_expect_success SYMLINKS 'a/b was resolved as symlink' '
 	test -h a/b
 '
 
-test_expect_failure 'do not lose untracked in merge (resolve)' '
+test_expect_success 'do not lose untracked in merge (resolve)' '
 	git reset --hard &&
 	git checkout baseline^0 &&
 	>a/b/c/e &&
