[RFC PATCH 2/7] path-walk: support `tree:0` filter

[Taylor Blau <me@ttaylorr.com>]

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.

Update Documentation/git-pack-objects.adoc to drop --filter from
the unconditional incompatibility list and call out the supported
subset (which already includes the filters added by Stolee's
earlier patches: blob:none, blob:limit, and sparse:oid).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-pack-objects.adoc | 10 +++----
 path-walk.c                         | 13 +++++++++
 t/t6601-path-walk.sh                | 45 +++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-pack-objects.adoc b/Documentation/git-pack-objects.adoc
index 8dea8259787..cfb5bc0ae16 100644
--- a/Documentation/git-pack-objects.adoc
+++ b/Documentation/git-pack-objects.adoc
@@ -402,11 +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`. Path-walk supports the
-`--filter=<spec>` forms `blob:none`, `blob:limit=<n>`, and
-`sparse:oid=<blob>`. Other filter forms fall back to the regular object
-traversal. The `--use-bitmap-index` option will be ignored in the
-presence of `--path-walk`.
+Incompatible with `--delta-islands`. Path-walk supports
+the `--filter=<spec>` forms `blob:none`, `blob:limit=<n>`,
+`sparse:oid=<blob>`, and `tree:0`. Other filter forms fall back to the
+regular object traversal. The `--use-bitmap-index` option will be
+ignored in the presence of `--path-walk`.
 
 
 DELTA ISLANDS
diff --git a/path-walk.c b/path-walk.c
index 700617ee2fe..36a1e5b967a 100644
--- a/path-walk.c
+++ b/path-walk.c
@@ -564,6 +564,19 @@ 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;
diff --git a/t/t6601-path-walk.sh b/t/t6601-path-walk.sh
index 520269dfc65..72e09211e63 100755
--- a/t/t6601-path-walk.sh
+++ b/t/t6601-path-walk.sh
@@ -590,6 +590,51 @@ 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:$(git rev-parse refs/tags/blob-tag)
+	blobs:0
+	commits:4
+	tags:7
+	trees:0
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'topic only, tree:0 filter' '
+	test-tool path-walk --filter=tree:0 -- topic >out &&
+
+	cat >expect <<-EOF &&
+	0:commit::$(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 &&
-- 
2.54.0.4.g6aa0d38a4ec