Show an error message when the user tries to set the commit flag or add trybots to a private issue.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 6a43b3d051e092173255fb74e14bca2472f3975f..f86a781765a4ab605459d63fbc8628a77f268dae 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2267,6 +2267,10 @@ def CMDtry(parser, args):
   if not cl.GetIssue():
     parser.error('Need to upload first')
 
+  props = cl.GetIssueProperties()
+  if props.get('private'):
+    parser.error('Cannot use trybots with private issue')
+
   if not options.name:
     options.name = cl.GetBranch()
 
@@ -2412,6 +2416,9 @@ def CMDset_commit(parser, args):
   if args:
     parser.error('Unrecognized args: %s' % ' '.join(args))
   cl = Changelist()
+  props = cl.GetIssueProperties()
+  if props.get('private'):
+    parser.error('Cannot set commit on private issue')
   cl.SetFlag('commit', '1')
   return 0