Add co_authored_by to merge commit templates

What does this MR do and why?

This merge requests add support for %{co_authored_by} variable in a merge commit templates. This solves #20421 (closed).

Variable expands to list of commit authors in a form:

Co-authored-by: Zane Doe <zdoe@example.com>
Co-authored-by: Alex Garcia <agarcia@example.com>

This format is often used by various tools to add more authors to a single commit, and would be most useful in a squash commit template. The format is already somewhat supported by gitlab, as the user mentioned that way is linked to in a commit view (see screenshots section).

A lot of complexity was added by the fact that the main commit author shouldn't be mentioned as a co-author. For merge commit, that means we should skip merging user (for non-merged MR we assume currently logged in user), while for squash commit we should skip MR author. We could simplify things a bit and future-proof this if we decided to ignore that and always include commit author as a co-author as well, so let me know if you'd rather have the simpler, but a bit less user-friendly version.

Screenshots or screen recordings

Docs:

The format is already somewhat supported by gitlab, as the user mentioned that way is linked to (it would just be a mailto link if no account would be tied to given email address):

How to set up and validate locally

1. Go to any project -> settings -> general.
2. Expand Merge Requests section.
3. Fill in Merge commit message template or Squash commit message template with new variable %{co_authored_by}.
4. Commit a new branch with couple commits created by different authors (with different emails).
5. Create a new MR from this branch.
6. Check that the default message in MR widget renders co authors correctly.

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	1 Message
	This merge request adds or changes documentation files. A review from the Technical Writing team before you merge is recommended. Reviews can happen after you merge.

Documentation review

The following files require a review from a technical writer:

• doc/user/project/merge_requests/commit_templates.md

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Please refer to the table below for assigning reviewers and maintainers suggested by Danger in the specified category:

Category	Reviewer	Maintainer
backend	Reuben Pereira (@rpereira2) (UTC+5.5)	Mark Chao (@lulalala) (UTC+8)
UX	Austin Regnery (@aregnery) (UTC-5)	No maintainer available

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added 1 commit

• bd0396d4 - Add merged_by and co_authors to merge commit templates

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mentioned in merge request !75639 (merged)

mentioned in issue #20421 (closed)

added UX label

@mvanremmerden I'm adding a UX label here as it feel to me that this is a user facing change. Please take a second look.

requested review from @pedroms

resolved all threads

added devopscreate groupcode review sectiondev labels

mentioned in issue gitlab-org/quality/triage-reports#5563 (closed)

@trakos thanks for this great contribution! I left a comment in !75639 (comment 760671957) that is also applicable to this MR.

FYI regarding the dependency of this MR with !75639 (merged), I marked that other MR as blocking this one:

removed review request for @pedroms

assigned to @trakos

added 2749 commits

• bd0396d4...23b81f0a - 2748 commits from branch gitlab-org:master
• 29f20b98 - Add co_authored_by to merge commit templates

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changed the description

changed title from Draft: Add co_authors to merge commit templates to Draft: Add co_authored_by to merge commit templates

approved this merge request

@pedroms, thanks for approving this merge request.

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marked this merge request as ready

Hi @eread @aqualls, please review this documentation Merge Request.

marked the checklist item I have evaluated the MR acceptance checklist for this MR. as completed

added documentation twtriaged labels

requested review from @aqualls

question (non-blocking): is it time to stop listing all the options?

As we keep adding more substitution options, should we stop listing them below the text area, and instead link out to the docs, like we do for markdown and quick actions?

added Technical Writing UI text docsfeature tw-weight3 twfinished typefeature labels and removed twtriaged label

@trakos I've left you a question, but it's non-blocking. The docs are fine. I'll approve and unassign myself. Adding @alexbuijs now.

changed milestone to %14.7

requested review from @alexbuijs and removed review request for @aqualls

added 1 commit

• 738e778b - Add co_authored_by to merge commit templates

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added 1 commit

• 143e577c - Add co_authored_by to merge commit templates

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changed the description

mentioned in issue #349193 (closed)

approved this merge request

requested review from @dbalexandre and removed review request for @alexbuijs

mentioned in merge request !77350 (merged)

resolved all threads

approved this merge request

enabled an automatic merge when the pipeline for 8a1cfb7b succeeds

@trakos Thanks for your contribution This LGTM. I triggered a new pipeline and set MWPS.

added backend label

merged

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mentioned in commit 316fbdcb

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mentioned in merge request !77758 (merged)

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mentioned in merge request kubitus-project/kubitus-installer!562 (merged)

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