rebase: choose merge base without unwanted revisions Previously, when there are 2 merge base candidates, we choose p1 blindly, which may make the merge result to have "unwanted content". This patch makes rebase smarter - choose a merge base that does not have "unwanted revs" if possible. Since we don't really have a good solution when there are "unwanted revs", abort in that case. Differential Revision: https://phab.mercurial-scm.org/D340