check-ignore -z: a single -z should apply to both input and output

check-ignore -z: a single -z should apply to both input and output

Unless a command has separate --nul-terminated-{input,output}
options, the --nul-terminated-records (-z) option should apply
to both input and output for consistency.  The caller knows that its
input paths may need to be protected for LF, and the program shows
these problematic paths to its output.

The code already did the right thing.  Only the help text needs
fixing.

Signed-off-by: Junio C Hamano <gitster@pobox.com>