git-fetch: allow updating the current branch in a bare repository.

git-fetch: allow updating the current branch in a bare repository.

Sometimes, people have only fetch access into a bare repository
that is used as a back-up location (or a distribution point) but
does not have a push access for networking reasons, e.g. one end
being behind a firewall, and updating the "current branch" in
such a case is perfectly fine.

This allows such a fetch without --update-head-ok, which is a
flag that should never be used by end users otherwise.

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