ls-tree: use a "struct options"

As a first step towards being able to turn this code into an API some
day let's change the "static" options in builtin/ls-tree.c into a
"struct ls_tree_options" that can be constructed dynamically without
the help of parse_options().

Because we're now using non-static variables for this we'll need to
clear_pathspec() at the end of cmd_ls_tree(), least various tests
start failing under SANITIZE=leak. The memory leak was already there
before, now it's just being brought to the surface.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Teng Long <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar