Consistently use perl from /usr/bin/ for scripts

Consistently use perl from /usr/bin/ for scripts

While the majority of scripts use '#!/usr/bin/perl', some use
'#!/usr/bin/env perl'. In the end there is no difference, because the
Makefile rewrites "#!.*perl" with "#!$PERL_PATH" in scripted
Porcelains before installing. Nevertheless, the second form can be
misleading, because it suggests that perl found first in $PATH will be
used.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>