topic: add a `exp….topic.linear-merge` option to allow some oedipus If this option is set to `from-branch`, a user can call `hg merge some-topic` from a bare branch even if `some-topic` is a direct descendant of the current working copy parents. This was previously denied if the changesets was on the same branch, since the result would be an "oedipus merge". Some user have been requesting this, and this type of merge is one of Gitlab standard way of merging a "Merge Request". That new option will unlock issue `heptapod#200` and make this mode available for those who wants it.