copies: make _backwardrenames() filter out copies by destination As shown by the test case, _backwardrenames() doesn't filter by the matcher. It doesn't show up in `hg status --copies` because that only prints files changed between the two commits. I wouldn't be surprised if some other command that replies on pathcopies() was broken before this patch, but I haven't bothered to check other commands. Differential Revision: https://phab.mercurial-scm.org/D5988