"tahoe cp -r --mutable": make mutable copy of immutable directories, vice versa

[warner]

Now that we have immutable directories (#607), we could use some CLI commands to take advantage of them. #828 is about having "tahoe backup" create immutable directories, but what if you want to convert those immutable directories into a form that you can modify again? tahoe cp seems like the most appropriate tool.

There are a couple of interesting forms of copying that could be done. (In each case, we're talking about directories, and not files.)

• original is immutable: make immutable copy (re-use same object)
• original is immutable: make mutable copy
• original is mutable: make mutable copy
• original is mutable: make immutable copy

The default for cp -r should be to use the same type of object: mutable-to-mutable or immutable-to-immutable (and of course, immutable-to-immutable means we just re-use the original dircap).

I think that tahoe cp should acquire a --mutable flag which tells it to always create mutable directories, even if the original was immutable. This would be used to convert your "tahoe backup" -created immutable directories into a form that you can modify.

Likewise, I think it should have a --immutable flag which tells it to always create immutable directories.

I think that files should be handled differently: basically the default should be mutable-to-mutable and immutable-gets-shared. If you copy with --immutable, then clearly that will trigger mutable-to-immutable (since immutable dirnodes are deep-immutable, so we can't fill them with mutable files). But if you copy with --mutable, I think we should create mutable dirnodes with immutable files. A separate flag (maybe --mutable-files) could be used if you really do want to turn all of your immutable files into mutable ones.