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, 13 insertions, 0 deletions

diff --git a/list-objects-filter-options.c b/list-objects-filter-options.c
index d259bdb2c1..e8da2e8581 100644
--- a/list-objects-filter-options.c
+++ b/list-objects-filter-options.c
@@ -49,6 +49,19 @@ static int gently_parse_list_objects_filter(
 			return 0;
 		}
 
+	} else if (skip_prefix(arg, "tree:", &v0)) {
+		unsigned long depth;
+		if (!git_parse_ulong(v0, &depth) || depth != 0) {
+			if (errbuf) {
+				strbuf_addstr(
+					errbuf,
+					_("only 'tree:0' is supported"));
+			}
+			return 1;
+		}
+		filter_options->choice = LOFC_TREE_NONE;
+		return 0;
+
 	} else if (skip_prefix(arg, "sparse:oid=", &v0)) {
 		struct object_context oc;
 		struct object_id sparse_oid;