Allow "-u" flag to tag signing

Allow "-u" flag to tag signing

The current "git tag -s" thing always uses the tagger name as the signing
user key, which is very irritating, since my key is under my email
address, but the tagger key obviously contains the actual machine name
too.

Now, I could just use "GIT_COMMITTER_EMAIL" and force it to be my real
email, but I actually think that it's nice to see which machine I use for
my work.

So rather than force my tagger ID to have to match the gpg key name, just
support the "-u" flag to "git tag" instead. It implicitly enables signing,
since it doesn't make any sense without it. Thus:

	git tag -u <gpg-key-name> <tag-name> [<tagged-object>]

will use the named gpg key for signing.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>