Fix impropery history graph trace in Mercurial

Summary: Fixes T11267. This data was coming back weird (in reverse order relative to the graph itself). Previously it worked OK anyway, but the new logic is a little more sensitive to the input.

Test Plan: Viewed a Mercurial repository with linear history, saw linear history.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11267

Differential Revision: https://secure.phabricator.com/D16229