evolution: make reporting of new unstable changesets optional This makes it possible to turn off the reporting of unstable changesets (e.g. "1 new orphan changesets"), just in case it's too slow for some users. Anyone who's been using the evolve extension has already been accepting the cost of the reporting, so this is just for the few users who have turned on obsmarkers but not been using the evolve extension (as I believe Facebook has). Differential Revision: https://phab.mercurial-scm.org/D1868