Merge branch 'ds/path-walk-filters'

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

Documentation/git-backfill.adoc

@@ -80,6 +80,10 @@ OPTIONS
 +
 You may also use commit-limiting options understood by
 linkgit:git-rev-list[1] such as `--first-parent`, `--since`, or pathspecs.
++
+Most `--filter=<spec>` options don't work with the purpose of
+`git backfill`, but the `sparse:<oid>` filter is integrated to provide a
+focused set of paths to download, distinct from the `--sparse` option.
 
 SEE ALSO
 --------

Documentation/git-pack-objects.adoc

@@ -402,9 +402,11 @@ will be automatically changed to version `1`.
 	of filenames that cause collisions in Git's default name-hash
 	algorithm.
 +
-Incompatible with `--delta-islands`, `--shallow`, or `--filter`. The
-`--use-bitmap-index` option will be ignored in the presence of
-`--path-walk.`
+Incompatible with `--delta-islands`. The `--use-bitmap-index` option is
+ignored in the presence of `--path-walk`. The `--path-walk` option
+supports the `--filter=<spec>` forms `blob:none`, `blob:limit=<n>`,
+`tree:0`, `object:type=<type>`, and `sparse:<oid>`. These supported filter
+types can be combined with the `combine:<spec>+<spec>` form.
 
 
 DELTA ISLANDS

Documentation/technical/api-path-walk.adoc

@@ -48,6 +48,13 @@ commits.
 	applications could disable some options to make it simpler to walk
 	the objects or to have fewer calls to `path_fn`.
 +
+Note that objects directly requested as pending objects (such as targets
+of lightweight tags or other ref tips) are always emitted to `path_fn`,
+even when the corresponding type flag is disabled. Only objects
+discovered during the tree walk are subject to these type filters. This
+ensures that objects specifically requested through the revision input
+are never silently dropped.
++
 While it is possible to walk only commits in this way, consumers would be
 better off using the revision walk API instead.