git-gui: Remove Pull menu and cleanup Branch/Fetch/Push menus.

git-gui: Remove Pull menu and cleanup Branch/Fetch/Push menus.

The Pull menu as it stands right now is a really horrible idea.  Most
users will have too many branches show up in this menu, and what with
the new globbing syntax for fetch entries we were offering up possible
merging that just isn't really valid.  So this menu is dead and will
be rewritten to support better merge capabilities.

The Branch menu shouldn't include a separator entry if there are no
branches, it just looks too damn weird.  This can happen in an initial
repository before any branches have been created and before the first
commit.

The Fetch and Push menus should just be organized around their own
menus rather than being given the menu to populate.

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