fmt-merge-msg: Add contents of merged tag in the merge message

fmt-merge-msg: Add contents of merged tag in the merge message

When a contributor asks the integrator to merge her history, a signed tag
can be a good vehicle to communicate the authenticity of the request while
conveying other information such as the purpose of the topic.

E.g. a signed tag "for-linus" can be created, and the integrator can run:

   $ git pull git://example.com/work.git/ for-linus

This would allow the integrator to run "git verify-tag FETCH_HEAD" to
validate the signed tag.

Update fmt-merge-msg so that it pre-fills the merge message template with
the body (but not signature) of the tag object to help the integrator write
a better merge message, in the same spirit as the existing merge.log summary
lines.

The message that comes from GPG signature validation is also included in
the merge message template to help the integrator verify it, but they are
prefixed with "#" to make them comments.

Signed-off-by: Junio C Hamano <gitster@pobox.com>