add 'tahoe sync' command, for bidirectional synchronization

[warner]

Peter pointed out that it would be really useful to have a tool that would perform a bidirectional sync between a local directory and a tahoe directory. You could establish a relationship between the two, then periodically sync them later. Multiple machines could sync to the same online target. The overall use case is to let a laptop user edit their files while they're on a plane, then have them uploaded automatically once they get online again, then be able to edit them from a desktop machine later.

Of course, this is really a version-control problem, and the general case (with multiple uncoordinated committers) is seriously non-trivial. But, the single-user infrequent-collision case is easier.

The basic idea would be that the host would have a table, with one row per local file, and each row would contain some sort of version identifier for the local file's contents, and a second identifier for the remote file contents, both as of the last sync. The Tahoe-side remote identifier could just be a readcap. The local identifier could be a full hash of the file, or it could be a timestamp+filesize optimization/abbreviation.

When the sync is performed, it checks the local file to see if it still matches the last-known-identifier, and it checks the remote file to see if it matches the last-known-identifier. If both are the same, no work needs to be done. If exactly one has changed, then that side is considered newer, and it is copied to the other side. If both have changed, we have a conflict, and the user is required to look at the two versions and make a decision. Once the file is processed, the table is updated with the new (identical) version identifiers.

A suitable UI for the conflict indication-and-resolution phase is necessary.