Survive extra "obsolete:" log output from the Mercurial evolve extension

Summary: See PHI502; see https://bugzilla.mozilla.org/show_bug.cgi?id=1448137.

Test Plan: I spent all of three minutes trying to install the evolve extension without success and gave up, but this probably does the right thing based on the example output in the Bugzilla issue.

Differential Revision: https://secure.phabricator.com/D19262