fast-export: handle all kinds of tag signatures

fast-export: handle all kinds of tag signatures

Currently the handle_tag() function in "builtin/fast-export.c" searches
only for "\n-----BEGIN PGP SIGNATURE-----\n" in the tag message to find
a tag signature.

This doesn't handle all kinds of OpenPGP signatures as some can start
with "-----BEGIN PGP MESSAGE-----" too, and this doesn't handle SSH and
X.509 signatures either as they use "-----BEGIN SSH SIGNATURE-----" and
"-----BEGIN SIGNED MESSAGE-----" respectively.

To handle all these kinds of tag signatures supported by Git, let's use
the parse_signed_buffer() function to properly find signatures in tag
messages.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>