The core information that we want here is about "does information from revision
X overwrite information in Y". To do so, we check is X is an ancestors of Y, but
this is an implementation details, they could be other ways. We update the
naming to clarify this (and align more with wording used in upcoming changesets.
For people curious about other ways: for example we could record the overwrite
graph as it happens and reuse that to check if X overwrite Y, without having to
do potential expensive is_ancestor call on the revision graph.