This method avoids breaking test_readonly. Just check if the dest file
exists, and avoid CoW probing when it does, so when CoW probing fails,
it can resume where the previous non-CoW copy left off.

If CoW has been probed already to work, delete the dest file
since a CoW copy will presumably work. It seems like it would be almost
as good to just skip CoW copying in this case too, but consider that the
dest file might have started to be copied from some other remote, not
using CoW, but CoW has been probed to work to copy from the current
place.

Sponsored-by: Dartmouth College's Datalad project