Use core.quotePath=false for more git diffs

This includes a minor refactor so that some gclient_scm methods
can all share the same core.quotePath specifier.

R=iannucci

Bug: 792302
Change-Id: Iaadf190f5c0666787cf7c2ccda88d6dba9aace9b
Reviewed-on: https://chromium-review.googlesource.com/823131
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
diff --git a/checkout.py b/checkout.py
index 4a59000..685d2f7 100644
--- a/checkout.py
+++ b/checkout.py
@@ -301,7 +301,7 @@
     if errors:
       raise PatchApplicationFailed(errors, verbose)
     found_files = self._check_output_git(
-        ['diff', '--ignore-submodules',
+        ['-c', 'core.quotePath=false', 'diff', '--ignore-submodules',
          '--name-only', '--staged']).splitlines(False)
     if sorted(patches.filenames) != sorted(found_files):
       extra_files = sorted(set(found_files) - set(patches.filenames))