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.