last-modified: document option '--max-depth'

last-modified: document option '--max-depth'

Option --max-depth is supported by git-last-modified(1), because it was
added to the diff machinery in a1dfa5448d (diff: teach tree-diff a
max-depth parameter, 2025-08-07).

This option is useful for everyday use of the git-last-modified(1)
command, so document it's existence in the man page.

To have it also appear in the help output of `git last-modified -h`,
move the handling of '--max-depth' to parse_options() in
builtin/last-modified.c itself. This prepares for the change in default
behavior in the next commit.

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