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