changelog: lazily parse files More of the same. Again, modest revset performance wins: author(mpm) 0.896565 0.822961 0.805156 desc(bug) 0.887169 0.847054 0.798101 date(2015) 0.878797 0.811613 0.786689 extra(rebase_source) 0.865446 0.797756 0.777408 author(mpm) or author(greg) 1.801832 1.668172 1.626547 author(mpm) or desc(bug) 1.812438 1.677608 1.613941 date(2015) or branch(default) 0.968276 0.896032 0.869017