Make sure heads/foo and tags/foo do not confuse things.

Make sure heads/foo and tags/foo do not confuse things.

When both heads/foo and tags/foo exist, get_sha1_basic("foo")
picked up the tag without complaining, which is quite confusing.
Make sure we require unambiguous form, "heads/foo" or "tags/foo"
in such cases.

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