filter-branch: don't use xargs -0

Some versions of xargs don't understand "-0"; fortunately in
this case we can get the same effect by using "git clean".
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -281,7 +281,7 @@ while read commit parents; do
 			die "Could not checkout the index"
 		# files that $commit removed are now still in the working tree;
 		# remove them, else they would be added again
-		git ls-files -z --others | xargs -0 rm -f
+		git clean -q -f -x
 		eval "$filter_tree" < /dev/null ||
 			die "tree filter failed: $filter_tree"