Do not enforce HEAD when a revision is not specified.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 29ee421acc431bcaa380eed0ef84c8bed1f7d5b0..f884eaa6c655afe2b42d1c9ba67e49e8aca953fc 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1598,6 +1598,10 @@ def CMDtry(parser, args):
   cl.RpcServer().trigger_try_jobs(
       cl.GetIssue(), patchset, options.name, options.clobber, options.revision,
       builders_and_tests)
+  print('Tried jobs on:')
+  length = max(len(builder) for builder in builders_and_tests)
+  for builder in sorted(builders_and_tests):
+    print '  %*s: %s' % (length, builder, ','.join(builders_and_tests[builder]))
   return 0