subrepo: fix merging of already merged subrepos (issue1986) This fixes a bug seen when merging a main repo which contains a subrepo when both repos have been merged before. Each repo (main and sub) has two branches, both of which have been merged before. In a subrepo, if the revision to merge to is an ancestor of the current rev, then the merge should be a noop. Test provided by Steve Losh.