New repository config item, repository.append_only, causes Borg to never
delete or append to existing data files. Hints and indices are handled
as before, old ones are deleted, because they can get quite large, but
are automatically reconstructed: no need to keep them.
When append_only is activated a file /path/to/repo/transactions/<NUMBER>
will be created for every commit.
Deleting all segments <NUMBER+1> and higher will rollback to that commit.
Note that this only influences Borg behaviour. Since repository config
can't be altered remotely (except for repository.key) this can't be
disabled when accessed remotely over SSH with "borg serve" as the
forced command.
This is only a feature to support the use case, and does not replace
appropriate file system permissions or monitoring.
Resolves#809