restore_env(): free the saved environment variable once we are done

restore_env(): free the saved environment variable once we are done

Just like we free orig_cwd, which is the value of the original
working directory saved in save_env_before_alias(), once we are
done with it, the contents of orig_env[] array, saved in the
save_env_before_alias() function should be freed; otherwise,
the second and subsequent calls to save/restore pair will leak
the memory allocated in save_env_before_alias().

Signed-off-by: Junio C Hamano <gitster@pobox.com>