Disable problematic test until we can get git pinned.

BUG=670678

Change-Id: I70f39618eea7dd6428d18e5ba60b539cdd57889c
Reviewed-on: https://chromium-review.googlesource.com/416357
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

tests/gclient_scm_test.py

@@ -490,6 +490,7 @@ class ManagedGitWrapperTestCase(BaseGitWrapperTestCase):
     sys.stdout.getvalue()
     sys.stdout.close()
 
+  @unittest.skip('Skipping until crbug.com/670884 is resolved.')
   def testUpdateLocked(self):
     if not self.enabled:
       return