more tr portability test script fixes

more tr portability test script fixes

Dealing with NULs is not always safe with tr. On Solaris,
incoming NULs are silently deleted by both the System V and
UCB versions of tr. When converting to NULs, the System V
version works fine, but the UCB version silently ignores the
request to convert the character.

This patch changes all instances of tr using NULs to use
"perl -pe 'y///'" instead.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>