git cl issue 0 should really clear issue.

git_cl.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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld and Gerrit."""
 
 from distutils.version import LooseVersion
@@ skipping 1237 matching lines @@
     """Set this branch's issue.  If issue isn't given, clears the issue."""
     issue_setting = self._codereview_impl.IssueSetting(self.GetBranch())
     codereview_setting = self._codereview_impl.GetCodereviewServerSetting()
     if issue:
       self.issue = issue
       RunGit(['config', issue_setting, str(issue)])
       codereview_server = self._codereview_impl.GetCodereviewServer()
       if codereview_server:
         RunGit(['config', codereview_setting, codereview_server])
     else:
-      current_issue = self.GetIssue()
-      if current_issue:
-        RunGit(['config', '--unset', issue_setting])
+      # Reset it regardless. It doesn't hurt.
+      config_settings = [issue_setting, self._codereview_impl.PatchsetSetting()]
+      for prop in (['last-upload-hash'] +
+                    self._codereview_impl._PostUnsetIssueProperties()):
+        config_settings.append('branch.%s.%s' % (self.GetBranch(), prop))
+      for setting in config_settings:
+        RunGit(['config', '--unset', setting], error_ok=True)
       self.issue = None
-      self.SetPatchset(None)
+      self.patchset = None
 
   def GetChange(self, upstream_branch, author):
     if not self.GitSanityChecks(upstream_branch):
       DieWithError('\nGit sanity check failure')
 
     root = settings.GetRelativeRoot()
     if not root:
       root = '.'
     absroot = os.path.abspath(root)
 
@@ skipping 217 matching lines @@
   @classmethod
   def IssueSettingSuffix(cls):
     """Returns name of git config setting which stores issue number for a given
     branch."""
     raise NotImplementedError()
 
   def PatchsetSetting(self):
     """Returns name of git config setting which stores issue number."""
     raise NotImplementedError()
 
+  def _PostUnsetIssueProperties(self):
+    """Which branch-specific properties to erase when unsettin issue."""
+    raise NotImplementedError()
+
   def GetRieveldObjForPresubmit(self):
     # This is an unfortunate Rietveld-embeddedness in presubmit.
     # For non-Rietveld codereviews, this probably should return a dummy object.
     raise NotImplementedError()
 
   def GetGerritObjForPresubmit(self):
     # None is valid return value, otherwise presubmit_support.GerritAccessor.
     return None
 
   def UpdateDescriptionRemote(self, description):
@@ skipping 221 matching lines @@
     """Return the git setting that stores this change's most recent patchset."""
     return 'branch.%s.rietveldpatchset' % self.GetBranch()
 
   def GetCodereviewServerSetting(self):
     """Returns the git setting that stores this change's rietveld server."""
     branch = self.GetBranch()
     if branch:
       return 'branch.%s.rietveldserver' % branch
     return None
 
+  def _PostUnsetIssueProperties(self):
+    """Which branch-specific properties to erase when unsetting issue."""
+    return ['rietveldserver']
+
   def GetRieveldObjForPresubmit(self):
     return self.RpcServer()
 
   def SetCQState(self, new_state):
     props = self.GetIssueProperties()
     if props.get('private'):
       DieWithError('Cannot set-commit on private issue')
 
     if new_state == _CQState.COMMIT:
       self.SetFlag('commit', '1')
@@ skipping 336 matching lines @@
     """Return the git setting that stores this change's most recent patchset."""
     return 'branch.%s.gerritpatchset' % self.GetBranch()
 
   def GetCodereviewServerSetting(self):
     """Returns the git setting that stores this change's Gerrit server."""
     branch = self.GetBranch()
     if branch:
       return 'branch.%s.gerritserver' % branch
     return None
 
+  def _PostUnsetIssueProperties(self):
+    """Which branch-specific properties to erase when unsetting issue."""
+    return [
+        'gerritserver',
+        'gerritsquashhash',
+    ]
+
   def GetRieveldObjForPresubmit(self):
     class ThisIsNotRietveldIssue(object):
       def __nonzero__(self):
         # This is a hack to make presubmit_support think that rietveld is not
         # defined, yet still ensure that calls directly result in a decent
         # exception message below.
         return False
 
       def __getattr__(self, attr):
         print(
@@ skipping 2781 matching lines @@
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   setup_color.init()
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)