include revision id in getdiff conduit call

Summary:
we need to know the revision id in order to generate
differential links.  It would be amazing if there existed some library
for object <-> network call mapping, and we could get all the
information about an object given some unique ID rather than having to
create a dictionary manually.  One can dream...

Test Plan:
1. setup on test phabricator instance
2. called differential.getdiff from client code with a valid diffid
3. verified that the correct revisionID was included

Reviewed By: epriestley
Reviewers: epriestley
CC: dpepper, aran, epriestley, mgummelt
Differential Revision: 795

src/applications/conduit/method/differential/getdiff/ConduitAPI_differential_getdiff_Method.php

@@ -79,6 +79,7 @@ class ConduitAPI_differential_getdiff_Method extends ConduitAPIMethod {
     $dict = array(
       'id' => $diff->getID(),
       'parent' => $diff->getParentRevisionID(),
+      'revisionID' => $diff->getRevisionID(),
       'sourceControlBaseRevision' => $diff->getSourceControlBaseRevision(),
       'sourceControlPath' => $diff->getSourceControlPath(),
       'changes' => array(),