path-walk: support `object:type` filter

The `object:type` filter accepts only objects of a single type; it is
the second member of the object-info-only filter family that bitmap
traversal already supports.

Like `blob:none` and `tree:0`, it can be evaluated with nothing more
than the object's type, which is exactly the granularity path-walk's
existing info->{commits,trees,blobs,tags} flags already control.

Map `LOFC_OBJECT_TYPE` in `prepare_filters()` by AND-ing each flag
against the filtered type. A single `object:type=X` filter
applied to the default info (all flags = 1) leaves `info->X = 1` and
all the others 0, which is what we want.

Using an AND rather than straight assignment prepares us for a
subsequent change to implement combined object filters.

The path-walk machinery is mostly already wired for the per-type
distinction:

 - `walk_path()` calls `path_fn` for a batch only when the corresponding
   `info->X` flag is set, so unwanted types are silently not reported.

 - `add_tree_entries()` skips tree entries of type `OBJ_BLOB` when
   `info->blobs` is unset, so we don't even allocate paths for them.

 - The commit-walk loop short-circuits the root-tree fetch when
   `!info->trees && !info->blobs`, so commit-only filters don't descend
   into trees at all.

But there are a couple of side effects of the "trees off, blobs on" case
that need fixing:

 1. 'setup_pending_objects()' previously skipped pending trees as soon
    as `info->trees` was zero. For 'object:type=blob' the call site
    needs those pending trees: a lightweight tag pointing to a tree, or
    an annotated tag whose peeled target is a tree, can both reach
    blobs that are otherwise unreachable from any commit's root tree.
    Loosen the gate to "if (!info->trees && !info->blobs) continue" and
    similarly retrieve the root_tree_list whenever either trees or
    blobs are wanted.

 2. The revision machinery's `handle_commit()` drops pending trees when
    `revs->tree_objects` is zero (see the 'OBJ_TREE' handler in
    revision.c), so by the time path-walk sees the pending list
    after `prepare_revision_walk()` the tree-bearing pendings would
    already be gone. Fix this by setting

        revs->tree_objects = info->trees || info->blobs

    so pending trees survive `prepare_revision_walk()` whenever we
    need to walk into them. Path-walk still resets tree_objects to
    zero immediately after `prepare_revision_walk()` returns, so the
    rev-walk itself never enumerates trees redundantly with
    path-walk's own descent.

Add coverage in t6601 for each of the four `object:type` values. The
'object:type=blob' test in particular asserts that file2 and child/file
(both reachable only through tag-pointed trees) show up in the output,
exercising the pending-tree fix.

Update Documentation/git-pack-objects.adoc to add object:type to
the list of supported --filter forms.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-objects.adoc

@@ -405,7 +405,7 @@ will be automatically changed to version `1`.
 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`, and `sparse:<oid>`.
+`tree:0`, `object:type=<type>`, and `sparse:<oid>`.
 
 
 DELTA ISLANDS

path-walk.c

@@ -382,7 +382,7 @@ static int walk_path(struct path_walk_context *ctx,
 			ret = ctx->info->path_fn(path, &filtered, list->type,
 						 ctx->info->path_fn_data);
 		oid_array_clear(&filtered);
-	} else if (path_is_for_direct_objects(path) ||
+	} else if ((!ctx->info->strict_types && path_is_for_direct_objects(path)) ||
 		   (list->type == OBJ_TREE && ctx->info->trees) ||
 		   (list->type == OBJ_BLOB && ctx->info->blobs) ||
 		   (list->type == OBJ_TAG && ctx->info->tags)) {
@@ -608,6 +608,17 @@ static int prepare_filters(struct path_walk_info *info,
 		}
 		return 1;
 
+	case LOFC_OBJECT_TYPE:
+		if (info) {
+			info->commits &= options->object_type == OBJ_COMMIT;
+			info->tags &= options->object_type == OBJ_TAG;
+			info->trees &= options->object_type == OBJ_TREE;
+			info->blobs &= options->object_type == OBJ_BLOB;
+			info->strict_types = 1;
+			list_objects_filter_release(options);
+		}
+		return 1;
+
 	case LOFC_SPARSE_OID:
 		if (info) {
 			struct object_id sparse_oid;

path-walk.h

@@ -47,6 +47,12 @@ struct path_walk_info {
 	int blobs;
 	int tags;
 
+	/**
+	 * If 'strict_types' is 0, then direct object requests will no longer
+	 * override the object type restrictions.
+	 */
+	int strict_types;
+
 	/**
 	 * If non-zero, specifies a maximum blob size. Blobs with a
 	 * size equal to or greater than this limit will not be

t/t6601-path-walk.sh

@@ -643,6 +643,90 @@ test_expect_success 'tree:1 filter is rejected' '
 	test_grep "tree:1 filter not supported by the path-walk API" err
 '
 
+test_expect_success 'all, object:type=commit filter' '
+	test-tool path-walk --filter=object:type=commit -- --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)
+	blobs:0
+	commits:4
+	tags:0
+	trees:0
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'all, object:type=tag filter' '
+	test-tool path-walk --filter=object:type=tag -- --all >out &&
+
+	cat >expect <<-EOF &&
+	0:tag:/tags:$(git rev-parse refs/tags/first)
+	0:tag:/tags:$(git rev-parse refs/tags/second.1)
+	0:tag:/tags:$(git rev-parse refs/tags/second.2)
+	0:tag:/tags:$(git rev-parse refs/tags/third)
+	0:tag:/tags:$(git rev-parse refs/tags/fourth)
+	0:tag:/tags:$(git rev-parse refs/tags/tree-tag)
+	0:tag:/tags:$(git rev-parse refs/tags/blob-tag)
+	blobs:0
+	commits:0
+	tags:7
+	trees:0
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'all, object:type=tree filter' '
+	test-tool path-walk --filter=object:type=tree -- --all >out &&
+
+	cat >expect <<-EOF &&
+	0:tree::$(git rev-parse topic^{tree})
+	0:tree::$(git rev-parse base^{tree})
+	0:tree::$(git rev-parse base~1^{tree})
+	0:tree::$(git rev-parse base~2^{tree})
+	1:tree:/tagged-trees:$(git rev-parse refs/tags/tree-tag^{})
+	1:tree:/tagged-trees:$(git rev-parse refs/tags/tree-tag2^{})
+	2:tree:a/:$(git rev-parse base:a)
+	3:tree:child/:$(git rev-parse refs/tags/tree-tag:child)
+	4:tree:left/:$(git rev-parse base:left)
+	4:tree:left/:$(git rev-parse base~2:left)
+	5:tree:right/:$(git rev-parse topic:right)
+	5:tree:right/:$(git rev-parse base~1:right)
+	5:tree:right/:$(git rev-parse base~2:right)
+	blobs:0
+	commits:0
+	tags:0
+	trees:13
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'all, object:type=blob filter' '
+	test-tool path-walk --filter=object:type=blob -- --all >out &&
+
+	cat >expect <<-EOF &&
+	0:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag^{})
+	0:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag2^{})
+	1:blob:a:$(git rev-parse base~2:a)
+	2:blob:left/b:$(git rev-parse base:left/b)
+	2:blob:left/b:$(git rev-parse base~2:left/b)
+	3:blob:right/c:$(git rev-parse base~2:right/c)
+	3:blob:right/c:$(git rev-parse topic:right/c)
+	4:blob:right/d:$(git rev-parse base~1:right/d)
+	blobs:8
+	commits:0
+	tags:0
+	trees:0
+	EOF
+
+	test_cmp_sorted expect out
+'
+
 test_expect_success 'setup sparse filter blob' '
 	# Cone-mode patterns: include root, exclude all dirs, include left/
 	cat >patterns <<-\EOF &&