agenda: Fix regression in handling of non-caterory filters
* lisp/org-agenda.el (org-agenda-filter-make-matcher): Combine filter
forms with `and' unless multiple positive categories are given.
06cf532f4 (org-agenda.el: Fix bug when using category filters,
2020-01-20) modified org-agenda-filter-make-matcher to group the form
with `or' rather than `and' for category filters. This logic was
tweaked again in a follow-up commit, 7e52b7661 (org-agenda: Fix logic
of `org-agenda-filter-make-matcher', 2020-02-19), which was supposed
to restrict the use of `or' to _multiple_ positive categories.
However, the follow-up commit incorrectly affected all filter types.
Avoid the check for non-category types.
Also, fix the regexp so that it matches whenever there are multiple
positive categories, not just a single one.
Reported-by: Jorge P. de Morais Neto <firstname.lastname@example.org>