last-modified: document option '-z'

last-modified: document option '-z'

The command git-last-modified(1) already recognizes the option '-z', and
similar to many other commands this will make the output NUL-terminated
instead of using newlines. Although, this option is missing from the
documentation, so add it.

In addition to that, 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.

Before, the parsing of option '-z' was done by diff_opt_parse(), which
is called by setup_revisions(). That would fill in `struct
diff_options::line_termination`, but that field was not used by the diff
machinery itself. Thus it makes more sense to have the handling of that
option completely in builtin/last-modified.c.

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