Fix depot_tools test for git 2.11

We were checking for output which has been dropped from 2.11.0, so loosen the
test a bit.

BUG=670678

Change-Id: Ic610c76ceed4ab42a3b9f2bb8b952a3689658cbf
Reviewed-on: https://chromium-review.googlesource.com/416402
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>

tests/gclient_scm_test.py

@@ -496,8 +496,7 @@ class ManagedGitWrapperTestCase(BaseGitWrapperTestCase):
     file_path = join(self.base_path, '.git', 'index.lock')
     with open(file_path, 'w'):
       pass
-    with self.assertRaisesRegexp(subprocess2.CalledProcessError,
-                                 'Unable to create.*/index.lock'):
+    with self.assertRaises(subprocess2.CalledProcessError):
       scm.update(options, (), [])
     sys.stdout.close()