git cl issue 0 should really clear issue.

tests/git_cl_test.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Unit tests for git_cl.py."""
 
 import os
 import StringIO
 import stat
@@ skipping 1435 matching lines @@
   def test_description_set_stdin(self):
     out = StringIO.StringIO()
     self.mock(git_cl.sys, 'stdout', out)
 
     self.mock(git_cl, 'Changelist', ChangelistMock)
     self.mock(git_cl.sys, 'stdin', StringIO.StringIO('hi \r\n\t there\n\nman'))
 
     self.assertEqual(0, git_cl.main(['description', '-n', '-']))
     self.assertEqual('hi\n\t there\n\nman', ChangelistMock.desc)
 
+  def test_cmd_issue_erase_existing(self):
+    out = StringIO.StringIO()
+    self.mock(git_cl.sys, 'stdout', out)
+    self.calls = [
+        ((['git', 'symbolic-ref', 'HEAD'],), 'feature'),
+        ((['git', 'config', 'branch.feature.rietveldissue'],), ''),
+        ((['git', 'config', 'branch.feature.gerritissue'],), '123'),
+        ((['git', 'config', '--unset', 'branch.feature.gerritissue'],), ''),
+        ((['git', 'config', '--unset', 'branch.feature.gerritpatchset'],), ''),
+        # Let this command raise exception (retcode=1) - it should be ignored.
+        ((['git', 'config', '--unset', 'branch.feature.last-upload-hash'],),
+         '', subprocess2.CalledProcessError(1, '', '', '', '')),
+        ((['git', 'config', '--unset', 'branch.feature.gerritserver'],), ''),
+        ((['git', 'config', '--unset', 'branch.feature.gerritsquashhash'],),
+         ''),
+    ]
+    self.assertEqual(0, git_cl.main(['issue', '0']))
+
 
 if __name__ == '__main__':
   git_cl.logging.basicConfig(
       level=git_cl.logging.DEBUG if '-v' in sys.argv else git_cl.logging.ERROR)
   unittest.main()