builtin/tag: add --format argument for tag -v

builtin/tag: add --format argument for tag -v

Adding --format to git tag -v mutes the default output of the GPG
verification and instead prints the formatted tag object.
This allows callers to cross-check the tagname from refs/tags with
the tagname from the tag object header upon GPG verification.

The callback function for for_each_tag_name() didn't allow callers to
pass custom data to their callback functions. Add a new opaque pointer
to each_tag_name_fn's parameter to allow this.

Signed-off-by: Lukas Puehringer <luk.puehringer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>