Improved probing when CoW copies can be made between files on the same
drive. Now supports CoW between BTRFS subvolumes. And, falls back to rsync
instead of using cp when CoW won't work, eg copies between repos on the
same EXT4 filesystem.
Rather than trying cp --reflink=always for each file copied to a remote,
it's tried once and if it fails it falls back to using rsync thereafter
for the lifetime of the Remote object. That avoids overhead of calling cp
which while small, will add up over a large number of files.
This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.
21ff5e1e5a