builtin/commit: refactor --trailer logic

builtin/commit: refactor --trailer logic

git-commit adds user trailers to the commit message by passing its
`--trailer` arguments to a child process running `git-interpret-trailers
--in-place`. This logic is broadly useful, not just for git-commit but
for other commands constructing message bodies (e.g. git-tag).

Let's move this logic from git-commit to a new function in the trailer
API, so that it can be re-used in other commands.

Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: John Passaro <john.a.passaro@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>