sha1_name: don't waste cycles in the @-parsing loop

The @-parsing loop unnecessarily checks for the sequence "@{" from
(len - 2) unnecessarily.  We can safely check from (len - 4).

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>