Merge branch 'ds/backfill'

Merge branch 'ds/backfill'

Lazy-loading missing files in a blobless clone on demand is costly
as it tends to be one-blob-at-a-time.  "git backfill" is introduced
to help bulk-download necessary files beforehand.

* ds/backfill:
  backfill: assume --sparse when sparse-checkout is enabled
  backfill: add --sparse option
  backfill: add --min-batch-size=<n> option
  backfill: basic functionality and tests
  backfill: add builtin boilerplate