Fix Mercurial commit history ordering

Summary:
See <https://discourse.phabricator-community.org/t/diffusion-observed-mercurial-repository-history-broken/825>.

In D18769, I rewrote this from using the `--branch` flag (which is unsafe and does not function on branches named `--config=x.y` and such).

However, this rewrite accidentally changed the result order, which impacted Mercurial commit hisotry lists and graphs. Swap the order of the constraints so we get newest-to-oldest again, as expected.

Test Plan: Viewed a Mercurial repository's history graph, saw sensible chronology after the patch.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18817

src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php

@@ -130,7 +130,7 @@ final class DiffusionHistoryQueryConduitAPIMethod
     } else {
       $path_arg = '';
       $revset_arg = hgsprintf(
-        'branch(%s) and reverse(ancestors(%s))',
+        'reverse(ancestors(%s)) and branch(%s)',
         $drequest->getBranch(),
         $commit_hash);
     }