git-filter-branch.sh: more portable tr usage: use \012, not \n.

I hesitate to suggest this, since GNU tr has accepted \n for 15 years,
but there are supposedly a few crufty vendor-supplied versions of tr still
in use.  Also, all of the other uses of tr-with-newline in git use \012.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>