last-modified: add option '-z' to help output

last-modified: add option '-z' to help output

The parsing of option '-z' is done by diff_opt_parse(), which is called
by setup_revisions(), and ends up filling in
`struct diff_options::line_termination`. But that field isn't used by
the diff machinery itself, only by builtin/last-modified.c to format
the output.

To have '-z' also appear in the help output of `git last-modified -h`,
move the handling of '-z' to parse_options() in builtin/last-modified.c
itself.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>