resolve: use the `parentchange` context manager to apply merge action
In an ideal world, we would not have to do that. However, we are miles away from
being ready to not have to do it.
So we add this context manager alongside a long comment. This will help use to
get to the point were have two distinct API with strict rules about when to call
them.
Differential Revision: https://phab.mercurial-scm.org/D11074
[hgk]
path = /usr/share/mercurial/hgk