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

project.py

@@ -2689,6 +2689,7 @@ class Project(object):
     def DiffZ(self, name, *args):
       cmd = [name]
       cmd.append('-z')
+      cmd.append('--ignore-submodules')
       cmd.extend(args)
       p = GitCommand(self._project,
                      cmd,