git-gui: Relocate the menu/transport menu code.

git-gui: Relocate the menu/transport menu code.

This code doesn't belong down in the main window UI creation,
its really part of the menu system and probably should be
located with it.  I'm moving it because I could not find
the code when I was looking for it earlier today, as it was
not where I expected it to be found.

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