list-objects-filter: implement filter tree:0

Teach list-objects the "tree:0" filter which allows for filtering
out all tree and blob objects (unless other objects are explicitly
specified by the user). The purpose of this patch is to allow smaller
partial clones.

The name of this filter - tree:0 - does not explicitly specify that
it also filters out all blobs, but this should not cause much confusion
because blobs are not at all useful without the trees that refer to
them.

I also considered only:commits as a name, but this is inaccurate because
it suggests that annotated tags are omitted, but actually they are
included.

The name "tree:0" allows later filtering based on depth, i.e. "tree:1"
would filter out all but the root tree and blobs. In order to avoid
confusion between 0 and capital O, the documentation was worded in a
somewhat round-about way that also hints at this future improvement to
the feature.

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/t/t5317-pack-objects-filter-objects.sh b/t/t5317-pack-objects-filter-objects.sh
index 9839b48..510d353 100755
--- a/t/t5317-pack-objects-filter-objects.sh
+++ b/t/t5317-pack-objects-filter-objects.sh
@@ -72,6 +72,34 @@ test_expect_success 'get an error for missing tree object' '
 	grep -q "bad tree object" bad_tree
 '
 
+test_expect_success 'setup for tests of tree:0' '
+	mkdir r1/subtree &&
+	echo "This is a file in a subtree" >r1/subtree/file &&
+	git -C r1 add subtree/file &&
+	git -C r1 commit -m subtree
+'
+
+test_expect_success 'verify tree:0 packfile has no blobs or trees' '
+	git -C r1 pack-objects --rev --stdout --filter=tree:0 >commitsonly.pack <<-EOF &&
+	HEAD
+	EOF
+	git -C r1 index-pack ../commitsonly.pack &&
+	git -C r1 verify-pack -v ../commitsonly.pack >objs &&
+	! grep -E "tree|blob" objs
+'
+
+test_expect_success 'grab tree directly when using tree:0' '
+	# We should get the tree specified directly but not its blobs or subtrees.
+	git -C r1 pack-objects --rev --stdout --filter=tree:0 >commitsonly.pack <<-EOF &&
+	HEAD:
+	EOF
+	git -C r1 index-pack ../commitsonly.pack &&
+	git -C r1 verify-pack -v ../commitsonly.pack >objs &&
+	awk "/tree|blob/{print \$1}" objs >trees_and_blobs &&
+	git -C r1 rev-parse HEAD: >expected &&
+	test_cmp expected trees_and_blobs
+'
+
 # Test blob:limit=<n>[kmg] filter.
 # We boundary test around the size parameter.  The filter is strictly less than
 # the value, so size 500 and 1000 should have the same results, but 1001 should