git-gui: Create missing branch head on initial commit.

git-gui: Create missing branch head on initial commit.

If we are making an initial commit our branch head did not exist when
we scanned for all heads during startup.  Consequently we won't have
it in our branch menu.  So force it to be put there after the ref was
created.

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