copies-rust: record "overwritten" information from both side on delete
With this change, we can ensure every (`dest`, `rev`) points to the same value,
making a lots of comparison simpler.
Differential Revision: https://phab.mercurial-scm.org/D9652
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)