Improve presubmit_support.py handling of git checkout.

tests/presubmit_unittest.py

Index: tests/presubmit_unittest.py
diff --git a/tests/presubmit_unittest.py b/tests/presubmit_unittest.py
index bfffb5b3bb651f489f6a54f582ec39e5b21acc05..d8f186b5cd3e75e510db31fcaa71ed22f1d3a30a 100755
--- a/tests/presubmit_unittest.py
+++ b/tests/presubmit_unittest.py
@@ -596,11 +596,14 @@ def CheckChangeOnCommit(input_api, output_api):
     self.UnMock(presubmit.os.path, 'exists')
     self.mox.StubOutWithMock(presubmit, 'DoPresubmitChecks')
     self.mox.StubOutWithMock(presubmit, 'ParseFiles')
-    presubmit.os.path.isdir(presubmit.os.path.join(self.fake_root_dir, '.git')
-        ).AndReturn(False)
     presubmit.os.path.isdir(presubmit.os.path.join(self.fake_root_dir, '.svn')
         ).AndReturn(False)
-    #presubmit.ParseFiles([], None).AndReturn([])
+    presubmit.os.path.isdir(presubmit.os.path.join(self.fake_root_dir, '.git')
+        ).AndReturn(False)
+    presubmit.subprocess.call(
+        ['git', 'rev-parse', '--show-cdup'],
+        cwd=self.fake_root_dir,
+        stdout=presubmit.subprocess.PIPE).AndReturn(1)
     presubmit.DoPresubmitChecks(mox.IgnoreArg(), False, False,
                                 mox.IgnoreArg(),
                                 mox.IgnoreArg(),