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.