To each the baseline collection (BC) for FOO in rev NNN, we end up asking the
FOO in HEAD for a pointer to the BC. If FOO doesn't exist in HEAD, then we can't
get to the BC and we can't do the copy.
One answer is a custom report (kill interop, good traffic usage).
Another answer is to navigate up the URL namespace until we find a parent that
exists in HEAD. Get to the right BC, and then the relpath is composed of the
up-navigations plus the baseline-relpath property. (decent interop (relpath is
non-interop anyway), poor network usage).
Note that this only occurs when somebody copies from an old revision and the
file is deleted in HEAD. Being relatively uncommon, we probably do not need to
worry about network usage, and should strive for max interop.
Blocker