path-walk: support tree:0 filter

The `tree:0` object filter omits all trees and blobs from the result,
keeping only commits and tags. Consequently, this filter type should
has a fairly straightforward integration with path-walk, as the decision
to include an object depends only on its type and does not depend on any
path-sensitive state.

Mapping it onto `path_walk_info` is direct: set `info->trees = 0` and
`info->blobs = 0` in `prepare_filters()` when the `LOFC_TREE_DEPTH`
choice is requested with depth zero. The existing code already plumbs
those flags through the rest of the walk:

 - 'walk_objects_by_path()' sets `revs->blob_objects = info->blobs` and
   `revs->tree_objects = info->trees` before `prepare_revision_walk()`,
   so the revision walk doesn't try to enumerate trees or blobs itself.

 - The commit-walk loop short-circuits the root-tree fetch with
   "if (!info->trees && !info->blobs) continue;", so we never even
   look up the root tree, let alone descend into it.

 - `setup_pending_objects()` skips pending trees and blobs based on
   the same flags.

This means the path-walk doesn't allocate or expand any tree structures
at all under `tree:0`, which matches the intended behavior of the
filter.

Non-zero tree-depth filters are not supported. Those depend on the depth
at which a tree is visited, which is a path-walk concept the filter
machinery doesn't currently share with the path-walk API. Reject them in
`prepare_filters()` with a helpful error and let pack-objects fall back
to the regular traversal, the same way it already does for unsupported
filters.

Add coverage in t6601 for both `--all` and a single-branch case to
confirm that no trees or blobs are emitted, and a separate test that
`tree:1` is rejected with the expected error message. Place the new
tests before "setup sparse filter blob" so they run on the original set
of refs, before the orphan branch that the sparse-tree tests create.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: ttaylorr

Documentation/git-pack-objects.adoc

@@ -404,8 +404,8 @@ will be automatically changed to version `1`.
 +
 Incompatible with `--delta-islands`. The `--use-bitmap-index` option is
 ignored in the presence of `--path-walk`. Whe `--path-walk` option
-supports the `--filter=<spec>` forms `blob:none`, `blob:limit=<n>`, and
-`sparse:<oid>`.
+supports the `--filter=<spec>` forms `blob:none`, `blob:limit=<n>`,
+`tree:0`, and `sparse:<oid>`.
 
 
 DELTA ISLANDS

path-walk.c

@@ -389,8 +389,8 @@ static int walk_path(struct path_walk_context *ctx,
 					ctx->info->path_fn_data);
 	}
 
-	/* Expand data for children. */
-	if (list->type == OBJ_TREE) {
+	/* Expand data for children, unless this is a direct-object path. */
+	if (list->type == OBJ_TREE && !path_is_for_direct_objects(path)) {
 		for (size_t i = 0; i < list->oids.nr; i++) {
 			ret |= add_tree_entries(ctx,
 					    path,
@@ -441,11 +441,14 @@ static int setup_pending_objects(struct path_walk_info *info,
 {
 	struct type_and_oid_list *tags = NULL;
 	struct type_and_oid_list *tagged_blobs = NULL;
+	struct type_and_oid_list *tagged_trees = NULL;
 	struct type_and_oid_list *root_tree_list = NULL;
 
 	if (info->tags)
 		CALLOC_ARRAY(tags, 1);
 	CALLOC_ARRAY(tagged_blobs, 1);
+	if (!info->trees)
+		CALLOC_ARRAY(tagged_trees, 1);
 	root_tree_list = strmap_get(&ctx->paths_to_lists, root_path);
 
 	/*
@@ -490,7 +493,15 @@ static int setup_pending_objects(struct path_walk_info *info,
 
 		switch (obj->type) {
 		case OBJ_TREE:
-			if (pending->path) {
+			if (tagged_trees) {
+				/*
+				 * Trees are disabled but pending trees
+				 * should still be emitted. Collect them
+				 * into a "/tagged-trees" list that
+				 * bypasses the object type filter.
+				 */
+				oid_array_append(&tagged_trees->oids, &obj->oid);
+			} else if (pending->path) {
 				char *path = *pending->path ? xstrfmt("%s/", pending->path)
 							    : xstrdup("");
 				add_path_to_list(ctx, path, OBJ_TREE, &obj->oid, 1);
@@ -534,6 +545,18 @@ static int setup_pending_objects(struct path_walk_info *info,
 			free(tagged_blobs);
 		}
 	}
+	if (tagged_trees) {
+		if (tagged_trees->oids.nr) {
+			const char *tagged_tree_path = "/tagged-trees";
+			tagged_trees->type = OBJ_TREE;
+			tagged_trees->maybe_interesting = 1;
+			strmap_put(&ctx->paths_to_lists, tagged_tree_path, tagged_trees);
+			push_to_stack(ctx, tagged_tree_path);
+		} else {
+			oid_array_clear(&tagged_trees->oids);
+			free(tagged_trees);
+		}
+	}
 	if (tags) {
 		if (tags->oids.nr) {
 			const char *tag_path = "/tags";
@@ -574,6 +597,19 @@ static int prepare_filters(struct path_walk_info *info,
 		}
 		return 1;
 
+	case LOFC_TREE_DEPTH:
+		if (options->tree_exclude_depth) {
+			error(_("tree:%lu filter not supported by the path-walk API"),
+			      options->tree_exclude_depth);
+			return 0;
+		}
+		if (info) {
+			info->trees = 0;
+			info->blobs = 0;
+			list_objects_filter_release(options);
+		}
+		return 1;
+
 	case LOFC_SPARSE_OID:
 		if (info) {
 			struct object_id sparse_oid;

t/t6601-path-walk.sh

@@ -596,6 +596,55 @@ test_expect_success 'all, blob:limit=3 filter' '
 	test_cmp_sorted expect out
 '
 
+test_expect_success 'all, tree:0 filter' '
+	test-tool path-walk --filter=tree:0 -- --all >out &&
+
+	cat >expect <<-EOF &&
+	0:commit::$(git rev-parse topic)
+	0:commit::$(git rev-parse base)
+	0:commit::$(git rev-parse base~1)
+	0:commit::$(git rev-parse base~2)
+	1:tag:/tags:$(git rev-parse refs/tags/first)
+	1:tag:/tags:$(git rev-parse refs/tags/second.1)
+	1:tag:/tags:$(git rev-parse refs/tags/second.2)
+	1:tag:/tags:$(git rev-parse refs/tags/third)
+	1:tag:/tags:$(git rev-parse refs/tags/fourth)
+	1:tag:/tags:$(git rev-parse refs/tags/tree-tag)
+	1:tag:/tags:$(git rev-parse refs/tags/blob-tag)
+	2:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag^{})
+	2:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag2^{})
+	3:tree:/tagged-trees:$(git rev-parse refs/tags/tree-tag^{tree})
+	3:tree:/tagged-trees:$(git rev-parse refs/tags/tree-tag2)
+	blobs:2
+	commits:4
+	tags:7
+	trees:2
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'topic only, tree:0 filter' '
+	test-tool path-walk --filter=tree:0 -- topic >out &&
+
+	cat >expect <<-EOF &&
+	0:commit::$(git rev-parse topic)
+	0:commit::$(git rev-parse base~1)
+	0:commit::$(git rev-parse base~2)
+	blobs:0
+	commits:3
+	tags:0
+	trees:0
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'tree:1 filter is rejected' '
+	test_must_fail test-tool path-walk --filter=tree:1 -- --all 2>err &&
+	test_grep "tree:1 filter not supported by the path-walk API" err
+'
+
 test_expect_success 'setup sparse filter blob' '
 	# Cone-mode patterns: include root, exclude all dirs, include left/
 	cat >patterns <<-\EOF &&