pull: move the cgresult logic in _pullchangeset Logical follow up of the previous changeset. The `_pullchangeset` is now responsible of setting the `pullop.cgresult`.

pull: move return code in the pull operation object With bundle2 we'll have multiple code path susceptible to be responsible for adding changeset during the pull. We move it to the pull operation to simplify this the coming logic. The one doing the adding the changegroup will set the value.

merge: use ancestor filename from planning phase instead of filectx ancestor test-merge-types.t changes a bit in flag merging. It relied on the implementation detail that 100% identical revlog entries are reused. The revlog reuse did that fctx.ancestor() saw an ancestor where there really not was one.