Fix diffusion.branchquery returning dictionary instead of array when branches are filtered out

Summary:
diffusion.branchquery can return dictionary instead of array if some branches are filtered out.
Eg.:

{
  "result": [
    {
      "shortName": "master",
      "commitIdentifier": "2817b0d8f79748ddfad0220c46d9b20bea34f460",
      "refType": "branch",
      "rawFields": {
        "objectname": "2817b0d8f79748ddfad0220c46d9b20bea34f460",
        "objecttype": "commit",

might become:

{
  "result": {
    "1": {
      "shortName": "master",
      "commitIdentifier": "2817b0d8f79748ddfad0220c46d9b20bea34f460",
      "refType": "branch",
      "rawFields": {
        "objectname": "2817b0d8f79748ddfad0220c46d9b20bea34f460",
        "objecttype": "commit",

Reproduction - find repository which has couple of branches, setup to track only some of them, execute diffusion.branchquery API call - result is dictionary instead of array

Test Plan: Apply patch, execution diffusion.branchquery call - result is no longer dictionary if it was one before

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D20973