copies: do not track backward copies, only renames (issue3739) The inverse of a rename is a rename, but the inverse of a copy is not a copy. Presenting it as such -- in particular, stuffing it into the same dict as real copies -- causes bugs because other code starts believing the inverse copies are real. The only test whose output changes is test-mv-cp-st-diff.t. When a backwards status -C command is run where a copy is involved, the inverse copy (which was hitherto presented as a real copy) is no longer displayed. Keeping track of inverse copies is useful in some situations -- composability of diffs, for example, since adding "a" followed by an inverse copy "b" to "a" is equivalent to a rename "b" to "a". However, representing them would require a more complex data structure than the same dict in which real copies are also stored.

copies: make debug messages more sensible The -> in debug messages is currently overloaded to mean both source to dest and dest to source. To fix this, we add explicit labels and make the arrow direction consistent.