Support branch offset/limit in Mercurial

Summary:
  - Support offset/limit as added by D2442, for Mercurial.
  - We just list everything and slice, no clear way to do better and this isn't a major perf issue.
  - No clear way to easily get by-update sorting, we can implement this by looking up revs if someone asks.
  - Bury some of the Git implementation details inside the Git query.

Test Plan:
  - Looked at a Git repo, clicked "View All Branches".
  - Looked at a Hg repo, clicked "View All Branches".
  - Limited page size to 1, made sure it limited properly.

Reviewers: aurelijus, btrahan, vrana

Reviewed By: aurelijus

CC: aran

Differential Revision: https://secure.phabricator.com/D2453

src/applications/diffusion/controller/branchtable/DiffusionBranchTableController.php

@@ -32,8 +32,7 @@ final class DiffusionBranchTableController extends DiffusionController {
     // TODO: Add support for branches that contain commit
     $query = DiffusionBranchQuery::newFromDiffusionRequest($drequest);
     $query->setOffset($pager->getOffset());
-    // we add 2 here, because of removed HEAD branch
-    $query->setLimit($pager->getPageSize() + 2);
+    $query->setLimit($pager->getPageSize() + 1);
     $branches = $query->loadBranches();
 
     $branches = $pager->sliceResults($branches);