Merge branch 'ds/path-walk-filters' into jch

Merge branch 'ds/path-walk-filters' into jch

The "git pack-objects --path-walk" traversal has been integrated
with several object filters, including blobless and sparse filters.

* ds/path-walk-filters:
  path-walk: support `combine` filter
  path-walk: support `object:type` filter
  path-walk: support `tree:0` filter
  t6601: tag otherwise-unreachable trees
  pack-objects: support sparse:oid filter with path-walk
  path-walk: add pl_sparse_trees to control tree pruning
  path-walk: support blob size limit filter
  backfill: die on incompatible filter options
  path-walk: support blobless filter
  path-walk: always emit directly-requested objects
  t/perf: add pack-objects filter and path-walk benchmark
  pack-objects: pass --objects with --path-walk
  t5620: make test work with path-walk var