Updating the filtering of Mercurial debug output

Summary:
With newer versions of Mercurial come newer debug messages which need filtered out.

1. In the scenario of Phabricator observing a hosted Mercurial repository which exists on a server in a multi-user environment it's possible that a repository computes branch cache at a tip revision which is not present. When this happens Mercurial will include in the debug output this information. This message indicates that the cache is going to be re-computed. See https://www.mercurial-scm.org/pipermail/mercurial/2014-June/047239.html.
2. Likely in some version with added or improved support for pager the debug info seems to indicate when a pager is being invoked for a command. This seems to print out regularly despite piping the stdout.
3. If the repository on Phabricator ever had the largefiles extension enabled then some additional details about "updated patterns" will print out.

Test Plan:
I verified an observed repository's history could be browsed, specifically the history of files which previously resulted in "Undefined offset: 1".

Added a unit test to check the results of filterMercurialDebugOutput().

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D21677