bbackupd only ever saves reverse diffs, corrupted files on store may be unrecoverable

[chris]

If the last version of a file on the store is corrupted, as reported by Magnus Homann, then the reverse diffs are all useless.

We should (add the option to) save a full copy of the file every so often, e.g. every 20 or 100 diffs, to allow recovery of some data in this case, at the expense of using more space on the store.