ls-tree: slightly refactor `show_tree()`

ls-tree: slightly refactor `show_tree()`

This is a non-functional change, we introduce an enum "ls_tree_cmdmode"
then use it to mark which columns to output.

This has the advantage of making the show_tree logic simpler and more
readable, as well as making it easier to extend new options (for example,
if we want to add a "--object-only" option, we just need to add a similar
"short-circuit logic in "show_tree()").

Helped-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Teng Long <redacted>
Signed-off-by: Junio C Hamano <redacted>