copies: filter out copies when target is not in destination manifest
When chaining a series of commits that copied a file with a series
that removed the destination file, we would still include the copy in
the result. Similar to the previous patch, I have checked that `hg
status --copies` is not affected by this bug, but I wouldn't be
surprised if some commands are.
Differential Revision: https://phab.mercurial-scm.org/D5989