builtin/tag: add --trailer option

builtin/tag: add --trailer option

git-tag supports interpreting trailers from an annotated tag message,
using --list --format="%(trailers)". However, the available methods to
add a trailer to a tag message (namely -F or --editor) are not as
ergonomic.

In a previous patch, we moved git-commit's implementation of its
--trailer option to the trailer.h API. Let's use that new function to
teach git-tag the same --trailer option, emulating as much of
git-commit's behavior as much as possible.

Helped-by: Patrick Steinhardt <ps@pks.im>
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>