prune: clean .git/shallow after pruning objects

This patch teaches "prune" to remove shallow roots that are no longer
reachable from any refs (e.g. when the relevant refs are removed).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/prune.c

@@ -170,5 +170,9 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 	s = mkpathdup("%s/pack", get_object_directory());
 	remove_temporary_files(s);
 	free(s);
+
+	if (is_repository_shallow())
+		prune_shallow(show_only);
+
 	return 0;
 }