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[0/3] format-patch: teach `--header-cmd`

Message ID cover.1709841147.git.code@khaugsbakk.name (mailing list archive)
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Series format-patch: teach `--header-cmd` | expand

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Kristoffer Haugsbakk March 7, 2024, 7:59 p.m. UTC 
(most of this is from the main commit with some elaboration in parts)

Teach git-format-patch(1) `--header-cmd` (with negation) and the
accompanying config variable `format.headerCmd` which allows the user to
add extra headers per-patch.

§ Motivation

format-patch knows `--add-header`. However, that seems most useful for
series-wide headers; you cannot really control what the header is like
per patch or specifically for the cover letter. To that end, teach
format-patch a new option which runs a command that has access to the
hash of the current commit (if it is a code patch) and the patch count
which is used for the patch files that this command outputs. Also
include an environment variable which tells the version of this API so
that the command can detect and error out in case the API changes.

This is inspired by `--header-cmd` of git-send-email(1).

§ Discussion

One may already get the commit hash from the commit by looking at the message id created by format-patch:

    Message-ID: <48c517ffb3dce2188aba5f0a2c1f4f9dc8df59f0.1709840255.git.code@khaugsbakk.name>

However that does not seem to be documented behavior, and relying on the
message id from various tools seems to have backfired before.[1]

It’s also more convenient to not have to parse any output from
format-patch.

One may also be interested in finding some information based on the
commit hash, not just simply to output the hash itself.