git-gui: Use git diff --submodule when available

Doing so is much faster and gives the same output.
Here are some numbers:

  $ time git submodule summary
  real	0m0.219s
  user	0m0.050s
  sys	0m0.111s

  $ time git diff --submodule
  real	0m0.012s
  user	0m0.003s
  sys	0m0.009s

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