git-gui: add a custom diff popup menu for submodules
To enhance user experience when dealing with submodules, a new diff popup
menu has been added for submodules. The "Show Less Context" and "Show More
Context" entries have been removed, as they don't make any sense for a
submodule summary. Four new entries are added to make it easier to easily
assess the changes commited in a subodule. These include starting a git gui
inside the submodule, using gitk to visualize the differences between two
commits, using gitk to view the whole submodule history until the current
commit and a gitk --all of the submodule.
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>