Add support for Co-authored-by: in commit

[Denton-L]

Since GitHub supports the Co-authored-by: trailer, I'd say it's a defacto standard.

Perhaps we could add an option such as git commit --coauthor="John Smith <js@something.com>" --coauthor="Smith John <sj@something.com>" which would automatically add the trailers. This should happen right before the SOB lines.

Things to consider:

• what other commands should get this option (anything supporting --signoff?)

• Do we want a config option for this? (Currently --signoff doesn't have one, does this deserve one?)

[To1ne]

I've sent a patch for this https://public-inbox.org/git/20191008074935.10972-1-toon@iotcl.com/T/#u

[Denton-L]

Individual trailer options will never be supported: https://lore.kernel.org/git/xmqqft8b6h52.fsf@gitster.c.googlers.com/

[phil-blain]

A more general option --trailer was added in that thread on the Git mailing list, which came out in Git 2.32.0:

git/Documentation/RelNotes/2.32.0.txt

Lines 55 to 57 in 4b6846d

	* "git commit" learned "--trailer <key>[=<value>]" option; together
	with the interpret-trailers command, this will make it easier to
	support custom trailers.

With this option you can do:

git commit --trailer "Co-authored-by: Jane Doe <their.email@domain.com>"

You can also setup a config variable to make it easier:

git config --global trailer.co.key "Co-authored-by"
git commit --trailer=co:"Jane Doe <their.email@domain.com>"

If your co-author has already authored some commits in the project's history, or you have another automated way to get their full name and email (LDAP for example), you can make that even shorter:

git config --global trailer.co.key "Co-authored-by"
git config --global alias.who = '!f () { git --no-pager -c mailmap.blob=origin/master:.mailmap log -1 --format="%aN <%aE>" --author="$1" -i origin/master;  }; f'
git config --global trailer.co.cmd "git who"
git commit --trailer=co:jane