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.