get_depends_on_patchset is actually a POST endpoint

Context= https://groups.google.com/a/chromium.org/d/msg/infra-dev/GNKEiqLqKPg/M_rjXvwvLQAJ

BUG=

Review URL: https://codereview.chromium.org/1315893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296430 0039d316-1c4b-4281-b951-d872f2087c98

rietveld.py

@@ -89,7 +89,7 @@ class Rietveld(object):
     url = '/%d/patchset/%d/get_depends_on_patchset' % (issue, patchset)
     resp = None
     try:
-      resp = json.loads(self.get(url))
+      resp = json.loads(self.post(url, []))
     except (urllib2.HTTPError, ValueError):
       # The get_depends_on_patchset endpoint does not exist on this Rietveld
       # instance yet. Ignore the error and proceed.