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/git_upstream_diff.py b/git_upstream_diff.py
index 72acb37..6e07c6d 100755
--- a/git_upstream_diff.py
+++ b/git_upstream_diff.py
@@ -34,7 +34,8 @@
     print 'fatal: No upstream configured for branch \'%s\'' % opts.branch
     return 1
 
-  cmd = [git.GIT_EXE, 'diff', '--patience', '-C', '-C']
+  cmd = [git.GIT_EXE, '-c', 'core.quotePath=false',
+         'diff', '--patience', '-C', '-C']
   if opts.wordwise:
     cmd += ['--word-diff=color', r'--word-diff-regex=(\w+|[^[:space:]])']
   cmd += [git.get_or_create_merge_base(opts.branch, par)]