date: fix parsing months Thanks nemo for discovering this on #mercurial IRC channel. Test Plan: Add a test. It fails before this patch: ``` + hg: parse error: invalid date: 'Feb 2018' + hg: parse error: invalid date: 'Apr 2018' + hg: parse error: invalid date: 'Jun 2018' + hg: parse error: invalid date: 'Sep 2018' + hg: parse error: invalid date: 'Nov 2018' ``` Differential Revision: https://phab.mercurial-scm.org/D2289