Incorporates the detailed explanation from Jeff King in
<20100410040959.GA11977@coredump.intra.peff.net> and fixes
the bug noted by Junio C Hamano in
<7vmxxc1i8g.fsf@alter.siamese.dyndns.org>.
Signed-off-by: Eric S. Raymond <esr@thyrsus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>