save: add test for --smaller, fix DESIGN document

save: add test for --smaller, fix DESIGN document

Add a test for --smaller, in particular showing that the actual --smaller
behaviour doesn't match what's described in the DESIGN file, which says:

    Another interesting trick is that you can skip backing up files even if
    IX_HASHVALID *isn't* set, as long as you have that file's sha1 in the
    repository.  What that means is you've chosen not to backup the latest
    version of that file; instead, your new backup set just contains the
    most-recently-known valid version of that file.  This is a good trick if you
    want to do frequent backups of smallish files and infrequent backups of
    large ones (as in 'bup save --smaller').  Each of your backups will be
    "complete," in that they contain all the small files and the large ones, but
    intermediate ones will just contain out-of-date copies of the large files.

This ("Each of your backups will be 'complete,' [...]") would seem to indicate
all files should be present, but in fact neither new nor old files are actually
saved by 'bup save --smaller'.

To avoid confusion, also update the DESIGN documentation here.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
(cherry picked from commit fb74d94974aeea5a2c78d3e32afe2aaaaf0d6f61)
Tested-by: Rob Browning <rlb@defaultvalue.org>