- A mirror download first copies the file out of the content cache when the
same revision is cached whole or block by block, so moving a file out of an
on-demand folder into a mirror costs no traffic for what was already read.
- `pdfs move a b c DEST` moves every source, reports each failure and exits
non-zero if any failed.
- Mirror-to-mirror moves rename with RENAME_NOREPLACE, so a name that
appeared at the destination after the check is never overwritten.
- The local copy a move out of a mirror drops is checked again just before
it is deleted; anything written into it meanwhile keeps the copy and is
uploaded as new by the next pass.