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>

git-filter-branch.sh

@@ -290,7 +290,7 @@ while read commit parents; do
 		eval "$filter_tree" < /dev/null ||
 			die "tree filter failed: $filter_tree"
 
-		git diff-index -r $commit | cut -f 2- | tr '\n' '\000' | \
+		git diff-index -r $commit | cut -f 2- | tr '\012' '\000' | \
 			xargs -0 git update-index --add --replace --remove
 		git ls-files -z --others | \
 			xargs -0 git update-index --add --replace --remove