parse-remote: trivial fix to allow refs/{heads,tags}/ spelled easier.

Earlier we always prefixed refs/heads to the token given to "git fetch"
(and "git pull") as refspec.  This was a mistake.  Allow them to be
spelled like "master:refs/tags/paulus" to mean "I want to fetch the
master there and store it as my local "paulus" tag.

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