mwindow: clean up pack map at shutdown

src/mwindow.c

@@ -41,6 +41,8 @@ int git_mwindow_files_init(void)
 	if (git__pack_cache)
 		return 0;
 
+	git__on_shutdown(git_mwindow_files_free);
+
 	return git_strmap_alloc(&git__pack_cache);
 }