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