git-gui: Allow creating a branch when none exists

git-gui: Allow creating a branch when none exists

If the user has no branches at all (their refs/heads/ is empty)
and they are on a detached HEAD we have a valid repository but
there are no branches to populate into the branch pulldown in
the create branch dialog.  Instead of erroring out we can skip
that part of the dialog, much like we do with tracking branches
or tags when the user doesn't have any.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>