builtin/verify-tag: add --format to verify-tag

builtin/verify-tag: add --format to verify-tag

Callers of verify-tag may want to cross-check the tagname from refs/tags
with the tagname from the tag object header upon GPG verification. This
is to avoid tag refs that point to an incorrect object.

Add a --format parameter to git verify-tag to print the formatted tag
object header in addition to or instead of the --verbose or --raw GPG
verification output.

Signed-off-by: Santiago Torres <santiago@nyu.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>