Fix git cl checkout.

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

Review URL: https://codereview.chromium.org/1877853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299827 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -4541,7 +4541,7 @@ def CMDcheckout(parser, args):
     return 1
 
   issue_arg = ParseIssueNumberArgument(args[0])
-  if issue_arg.valid:
+  if not issue_arg.valid:
     parser.print_help()
     return 1
   target_issue = issue_arg.issue