Ignore submodules when calculating 'git diff-files'

This allows projects to include submodules inside of
projects that use repo without repo incorrectly believing
the area is dirty just because a submodule has updates.
This is in line with git porcelain commands which generally
require a commandline flag to include submodules (git add,
git rebase).

Change-Id: Ide8a292162a42ab35145b5c4ca8ca0d020cdfe81
diff --git a/project.py b/project.py
index f9eb7a2..65bc3a0 100755
--- a/project.py
+++ b/project.py
@@ -2689,6 +2689,7 @@
     def DiffZ(self, name, *args):
       cmd = [name]
+      cmd.append('--ignore-submodules')
       p = GitCommand(self._project,