Ignore git submodules when calculating dirty files.

This is in keeping with git's default behavior for other porcelain
commands. It also makes it more tenable for projects to introduce
submodules instead of repo manifests without breaking git-cl upload
and similar tools.

Bug: 958499
Test: Run locally
Change-Id: Id9595271bf89aa8ba928026501a4c9487e5abf8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1592572
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Eli Ribble <eliribble@chromium.org>

git_common.py

@@ -777,7 +777,7 @@ def set_config(option, value, scope='local'):
 def get_dirty_files():
   # Make sure index is up-to-date before running diff-index.
   run_with_retcode('update-index', '--refresh', '-q')
-  return run('diff-index', '--name-status', 'HEAD')
+  return run('diff-index', '--ignore-submodules', '--name-status', 'HEAD')
 
 
 def is_dirty_git_tree(cmd):