Handle empty output from `hg --debug branches` in the parser

Summary: Ref T1493. Also consolidate this a bit more.

Test Plan: Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1493

Differential Revision: https://secure.phabricator.com/D7481
Merge branch 'master' of http://github.com/facebook/arcanist