Do not enforce HEAD when a revision is not specified.

rietveld.py

Index: rietveld.py
diff --git a/rietveld.py b/rietveld.py
index 94f7c6f524ce3212e02d7496960a242a61af1563..e0bd7a54adc91b68155ab209ca83c7e7159c52e0 100644
--- a/rietveld.py
+++ b/rietveld.py
@@ -315,10 +315,11 @@ class Rietveld(object):
     params = [
       ('reason', reason),
       ('clobber', 'True' if clobber else 'False'),
-      ('revision', revision if revision else 'HEAD'),
       ('builders', json.dumps(builders_and_tests)),
       ('xsrf_token', self.xsrf_token()),
     ]
+    if revision:
+      params.append(('revision', revision))
     return self.post('/%d/try/%d' % (issue, patchset), params)
 
   def get_pending_try_jobs(self, cursor=None, limit=100):