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
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors