buildbucket: support unique change urls

scripts/master/buildbucket/buildbot_gateway.py

Index: scripts/master/buildbucket/buildbot_gateway.py
diff --git a/scripts/master/buildbucket/buildbot_gateway.py b/scripts/master/buildbucket/buildbot_gateway.py
index 2bd422475627e5149ed6eeb33e958c0d161c4967..c465e3bd2613660bee92c4bed85c1a60a9658224 100644
--- a/scripts/master/buildbucket/buildbot_gateway.py
+++ b/scripts/master/buildbucket/buildbot_gateway.py
@@ -90,6 +90,14 @@ class BuildbotGateway(object):
       return [row.changeid for row in conn.execute(q)]
     return self.master.db.pool.do(find)
 
+  def find_changes_by_revlink(self, revlink):
+    """Searches for Changes in database by |revlink| and returns change ids."""
+    def find(conn):
+      table = self.master.db.model.changes
+      q = sa.select([table.c.changeid]).where(table.c.revlink == revlink)
+      return [row.changeid for row in conn.execute(q)]
+    return self.master.db.pool.do(find)
+
   @inlineCallbacks
   def get_change_by_id(self, change_id):
     """Returns buildot.changes.changes.Change as Deferred for |change_id|."""