git-gui: Add a special diff popup menu for submodules

git-gui: Add a special diff popup menu for submodules

To make it easier for users to deal with submodules, a special 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 the
top of the popup menu to gain access to more detailed information
about the changes in a submodule than the plain summary does offer.

These are:
- "Visualize These Changes In The Submodule"
  starts gitk showing the selected commit range

- "Visualize These Changes In The Submodule"
  starts gitk showing the whole submodule history of the current branch

- "Visualize All Branch History In The Submodule"
  starts gitk --all in the submodule

- "Start git gui In The Submodule"
  guess what :-)

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>