add 'tahoe sync' command, use backupdb

[warner]

It would be nice to have a CLI tool which does a minimum-effort copy, from local disk into a Tahoe directory. This tool should behave like "rsync -a --delete": when done, the target directory should look exactly like the local disk's directory. By running the tool on a periodic basis, the Tahoe directory will contain a single most-recent-version backup of the local disk.

tahoe sync ~/music tahoe:my-music

To make this as fast as possible, the default mode will use two assumptions:

• the target Tahoe directory has not been changed by other parties since the last invocation of 'tahoe sync'
• any file changes will modify either their timestamp or filesize

Both assumptions can be disabled with argv flags, at the expense of doing more work. If both assumptions are accepted, then a null backup should require no network traffic and no file reads (only directory reads).

This command would use the 'backupdb', stored in ~/.tahoe/private/backupdb, as discussed in this thread: ​http://allmydata.org/pipermail/tahoe-dev/2008-May/000620.html . The backupdb would allow the client to quickly determine which files and directories have already been copied.

The proposed "tahoe backup" command would use the same backupdb.