### Please describe the problem.

The tahoe-lafs remote has no built-in way to perform the repair operation.
This results to data loss if expiration is enabled on the Tahoe grid.

For the current tahoe-lafs release (1.10.0), the only way storage space is freed
is via garbage collection. Garbage collection removes shares whose lease has expired.
Data loss will occur if leases are not periodically renewed via
"tahoe repair --add-lease WRITECAP".

The current implementation of the Tahoe remote in git-annex does not offer a way to
run lease renewal, and cannot be used on grids where GC is enabled. (GC is not enabled
in the default configuration, but on private grids it is a sensible option.)

One way renewal could be made easier to do is to add the uploaded files to a directory
in Tahoe, so that the leases could be easily updated if the directory writecap is known,
without needing to go through the full list of writecaps for each file stored.

### What steps will reproduce the problem?

1. Use tahoe remote on a tahoe grid where GC is enabled.

2. After GC expiration period, data loss ensues.

### What version of git-annex are you using? On what operating system?

Seems to affect current git master (as of 2014-08-24).
