For `restic backup`, support new flags
`--changed-files-from-verbatim` and `--changed-files-from-raw` to
read the files/dirs that actually have changed from a file (or
multiple files). Directories that don't (directly or indirectly)
contain any changed files/dirs will reuse the corresponding subtree
of the parent snapshot.
This option is useful for higher-level backup tools which use
restic as a backend but have their own mechanism of figuring out
which files have changed (e.g., using zfs or btrfs diff tools).
We require to explicitly pass `--parent` as a protection mechanism
in order to make sure the higher-level backup tool and restic agree
on the parent snapshot. Though the caller can circumvent this
protection mechanism by passing `--parent latest`.
Caveat: since device IDs are unstable (across reboots or across
different zfs/btrfs snapshots of the same subvolume), the parent
snapshot and current snapshot might have mismatching device IDs.
In this case, the feature will still reuse subtrees of the parent
snapshot (under the conditions mentioned above), so we end up with
a snapshot that contains subtrees with different `device_id`
values, even if there was only a single mountpoint in play.
For now, we could simply document this caveat and discourage users
who rely on correct restoration of hardlinks from using this
feature. When https://github.com/restic/restic/issues/3041 is
properly fixed in the future, then this caveat probably goes away,
too.
The idea for this feature emerged here:
https://github.com/restic/restic/issues/1502#issuecomment-1721956623
The ioutil functions are deprecated since Go 1.17 and only wrap another
library function. Thus directly call the underlying function.
This commit only mechanically replaces the function calls.
