Change rietveld.py to use GET on get_depends_on_patchset

Bug: 770327
Change-Id: I9dd0ed0be0761de4968136f563e8986b979c35cf
Reviewed-on: https://chromium-review.googlesource.com/723824
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Ravi Mistry <rmistry@chromium.org>

rietveld.py

@@ -93,7 +93,7 @@ class Rietveld(object):
     url = '/%d/patchset/%d/get_depends_on_patchset' % (issue, patchset)
     resp = None
     try:
-      resp = json.loads(self.post(url, []))
+      resp = json.loads(self.get(url))
     except (urllib2.HTTPError, ValueError):
       # The get_depends_on_patchset endpoint does not exist on this Rietveld
       # instance yet. Ignore the error and proceed.