copies-rust: rename Oracle.is_ancestor to Oracle.is_overwrite 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. Differential Revision: https://phab.mercurial-scm.org/D9496