git cl checkout: reall fix it.

TBR=machenbach@chromium.org,andybons@chromium.org
BUG=579182

Review URL: https://codereview.chromium.org/1874253003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299829 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -4544,7 +4544,7 @@ def CMDcheckout(parser, args):
   if not issue_arg.valid:
     parser.print_help()
     return 1
-  target_issue = issue_arg.issue
+  target_issue = str(issue_arg.issue)
 
   key_and_issues = [x.split() for x in RunGit(
       ['config', '--local', '--get-regexp', r'branch\..*\.rietveldissue'])