git-gui: display summary when showing diff of a submodule

As it is hard to say what changed in a submodule by looking at the hashes,
let's show the colored submodule summary instead.

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