pull: Add a --dry-run option for static deltas

pull: Add a --dry-run option for static deltas

One of the design goals with deltas was not just wire efficiency,
but also having all the data up front about how much data would
be transferred before starting.

Let's expose that better by adding a `dry-run` option to the pull API.
This requires static deltas to be useful.  Basically we simply call
the progress callback once with the data from the superblock.