sparse-checkout: avoid using internal API of unpack-trees

sparse-checkout: avoid using internal API of unpack-trees

struct unpack_trees_options has the following field and comment:

	struct pattern_list *pl; /* for internal use */

Despite the internal-use comment, commit e091228e17 ("sparse-checkout:
update working directory in-process", 2019-11-21) starting setting this
field from an external caller.  At the time, the only way around that
would have been to modify unpack_trees() to take an extra pattern_list
argument, and there's a lot of callers of that function.  However, when
we split update_sparsity() off as a separate function, with
sparse-checkout being the sole caller, the need to update other callers
went away.  Fix this API problem by adding a pattern_list argument to
update_sparsity() and stop setting the internal o.pl field directly.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>