make the sample pre-commit hook script reject names with newlines, too

make the sample pre-commit hook script reject names with newlines, too

The sample pre-commit hook script would fail to reject a file name like
"a\nb" because of the way newlines are handled in "$(...)".  Adjust the
test to count filtered bytes and require there be 0.  Also print all
diagnostics to standard error, not stdout, so they will actually be seen.

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