This handles the case where we upload a new tahoe directory for a
previously-processed local directory, possibly creating a new dircap (if the
metadata had changed). Now we replace the old dirhash->dircap record. The
previous behavior left the old record in place (with the old dircap and
timestamps), so we'd never stop creating new directories and never converge
on a null backup.
* backups now share dirnodes with any previous backup, in any location,
so renames and moves are handled very efficiently
* "tahoe backup" no longer bothers reading the previous snapshot
* if you switch grids, you should delete ~/.tahoe/private/backupdb.sqlite,
to force new uploads of all files and directories
