git-gui: Don't offer tracking branches if none exist.

git-gui: Don't offer tracking branches if none exist.

I refactored the common code related to tracking branch listing into
a new procedure all_tracking_branches.  This saves a few lines and
should make the create and delete dialogs easier to maintain.

We now don't offer a radio button to create from a tracking branch
or merge-check a tracking branch if there are no tracking branches
known to git-gui.  This prevents us from creating an empty option
list and letting the user try to shoot themselves in the foot by
asking us to work against an empty initial revision.

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