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

Author: Arturas Moskvinas arturas@uber.com

src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php

@@ -128,6 +128,8 @@ private function processBranchRefs(ConduitAPIRequest $request, array $refs) {
       $refs = array_slice($refs, 0, $limit);
     }
 
+    $refs = array_values($refs);
+
     return mpull($refs, 'toDictionary');
   }