upgrade: start moving the "to be happening" data in a dedicated object
The upgrade code has a lot of logic to determine which action needs to be
performed depending of various element (sometimes depending from each other). It
would be nice to have a consistent object representing this. That could be
cleanly passed and avoid some logic duplication.
So we create this object as a start.
Differential Revision: https://phab.mercurial-scm.org/D9487