copies: rewrite copy detection for non-merge users The existing copy detection API was designed with merge in mind and was ill-suited for doing status/diff. The new pathcopies implementation gives more accurate, easier to use results for comparing two revisions, and is much simpler to understand. Test notes: - test-mv-cp-st.t results finds more renames in the reverse direction now - test-mq-merge.t was always wrong and duplicated a copy in diff that was already present in one of the parent revisions