Make "git cl issue 0" idempotent.

Currently, running "git cl issue 0" in a client prints a confusing error
message if there's no issue assigned. This changes the code so that the issue
number isn't "unset" if it's not currently set.

BUG=

Review URL: https://codereview.chromium.org/32193016

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@230499 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -702,7 +702,9 @@ or verify this branch is set up to track another (via the --track argument to
       if self.rietveld_server:
         RunGit(['config', self._RietveldServer(), self.rietveld_server])
     else:
-      RunGit(['config', '--unset', self._IssueSetting()])
+      current_issue = self.GetIssue()
+      if current_issue:
+        RunGit(['config', '--unset', self._IssueSetting()])
       self.issue = None
       self.SetPatchset(None)