Copy with a reflink when exporting a tree to a directory special remote

Copy with a reflink when exporting a tree to a directory special remote

Remote.Directory makes a temp file, then calls this, and since the temp
file exists, it prevented probing if CoW works.

Note that deleting the empty file does mean there's a small window for a
race. If another process is also exporting to the remote, that could let it
make the same temp file. However, the temp filename actually has the
processes's pid in it, which avoids that being a problem.

This may have been a reversion caused by commits around
63d508e8855b2e61a725c906ea17d1c7f4a2e125, but I haven't gone back and
tested to be sure. The directory special remote had supposedly supported
CoW for this going back to about half a year before that.

Sponsored-by: Graham Spencer on Patreon