git-push: accept tag <tag> as advertised.

git-push: accept tag <tag> as advertised.

The documentation talked about "git push $URL tag <tag>" as a short-hand
for refs/tags/<tag>:refs/tags/<tag> for a long time but that was never
the case (the short-hand was for "git fetch").  Instead of fixing the
documentation, just add a bit of code to match it since it is easy to do
and would make it more consistent.

Signed-off-by: Junio C Hamano <junkio@cox.net>