Summary:
Add new --sample-offset argument so that in combination with the existing --sample-rate the whole repo can be sampled in slices
For --sample-rate=N, this allows us to scrub or corpus dump 1/Nth of the repo a time, which is particularly useful for corpus dumping on machines with limited disk.
Also factored out the sampling args construction as 3 of the 4 walk variants use them (only validate does not)
Reviewed By: krallin
Differential Revision: D21158486
fbshipit-source-id: 94f98ceb71c22e0e9d368a563cdb04225b6fc459
