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