ls-tree: fix "--name-only" and "--long" combined use bug

ls-tree: fix "--name-only" and "--long" combined use bug

If we execute "git ls-tree" with combined "--name-only" and "--long"
, only the pathname will be printed, the size is omitted (the original
discoverer was Peff in [1]).

This commit fix this issue by using `OPT_CMDMODE()` instead to make both
of them mutually exclusive.

[1] https://public-inbox.org/git/YZK0MKCYAJmG+pSU@coredump.intra.peff.net/

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>