Fix default commits value in diffidtonode

Summary:
The bad type on the default value is causing crashes when there are no "local:commits" fields on a phabricator version.

Created from Diffusion's 'Open in Editor' feature.

Reviewed By: chadaustin, grakkpl

Differential Revision: D22259485

fbshipit-source-id: b11a82b2b099aea373734af96cbd4834f5c5dcbc

eden/scm/edenscm/hgext/phrevset.py

@@ -281,7 +281,7 @@ def diffidtonode(repo, diffid):
 
         try:
             props = resp["phabricator_version_properties"]["edges"]
-            commits = []
+            commits = {}
             for prop in props:
                 if prop["node"]["property_name"] == "local:commits":
                     commits = json.loads(prop["node"]["property_value"])