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