log: --show-signature

This teaches the "log" family of commands to pass the GPG signature in the
commit objects to "gpg --verify" via the verify_signed_buffer() interface
used to verify signed tag objects. E.g.

    $ git show --show-signature -s HEAD

shows GPG output in the header part of the output.

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

revision.h

@@ -110,6 +110,7 @@ struct rev_info {
 			show_merge:1,
 			show_notes:1,
 			show_notes_given:1,
+			show_signature:1,
 			pretty_given:1,
 			abbrev_commit:1,
 			abbrev_commit_given:1,