builtin/cat-file: support "object:type=" objects filter

Implement support for the "object:type=" filter in git-cat-file(1),
which causes us to omit all objects that don't match the provided object
type.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

Documentation/git-cat-file.adoc

@@ -96,6 +96,9 @@ The form '--filter=blob:limit=<n>[kmg]' omits blobs of size at least n
 bytes or units.  n may be zero.  The suffixes k, m, and g can be used to name
 units in KiB, MiB, or GiB.  For example, 'blob:limit=1k' is the same as
 'blob:limit=1024'.
++
+The form '--filter=object:type=(tag|commit|tree|blob)' omits all objects which
+are not of the requested type.
 
 --path=<path>::
 	For use with `--textconv` or `--filters`, to allow specifying an object

builtin/cat-file.c

@@ -484,7 +484,8 @@ static void batch_object_write(const char *obj_name,
 
 		if (use_mailmap ||
 		    opt->objects_filter.choice == LOFC_BLOB_NONE ||
-		    opt->objects_filter.choice == LOFC_BLOB_LIMIT)
+		    opt->objects_filter.choice == LOFC_BLOB_LIMIT ||
+		    opt->objects_filter.choice == LOFC_OBJECT_TYPE)
 			data->info.typep = &data->type;
 		if (opt->objects_filter.choice == LOFC_BLOB_LIMIT)
 			data->info.sizep = &data->size;
@@ -521,6 +522,14 @@ static void batch_object_write(const char *obj_name,
 				return;
 			}
 			break;
+		case LOFC_OBJECT_TYPE:
+			if (data->type != opt->objects_filter.object_type) {
+				if (!opt->all_objects)
+					report_object_status(opt, obj_name,
+							     &data->oid, "excluded");
+				return;
+			}
+			break;
 		default:
 			BUG("unsupported objects filter");
 		}
@@ -1062,6 +1071,7 @@ int cmd_cat_file(int argc,
 		break;
 	case LOFC_BLOB_NONE:
 	case LOFC_BLOB_LIMIT:
+	case LOFC_OBJECT_TYPE:
 		if (!batch.enabled)
 			usage(_("objects filter only supported in batch mode"));
 		break;

t/t1006-cat-file.sh

@@ -1388,7 +1388,7 @@ test_expect_success 'objects filter with unknown option' '
 	test_cmp expect err
 '
 
-for option in object:type=tag sparse:oid=1234 tree:1 sparse:path=x
+for option in sparse:oid=1234 tree:1 sparse:path=x
 do
 	test_expect_success "objects filter with unsupported option $option" '
 		case "$option" in
@@ -1447,5 +1447,9 @@ test_objects_filter "blob:limit=1"
 test_objects_filter "blob:limit=500"
 test_objects_filter "blob:limit=1000"
 test_objects_filter "blob:limit=1k"
+test_objects_filter "object:type=blob"
+test_objects_filter "object:type=commit"
+test_objects_filter "object:type=tag"
+test_objects_filter "object:type=tree"
 
 test_done