R1:a989dd181dd1

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…