[Bug] org-global-tags-completion-table does not include tags from buffers

[Matt Lundin <mdl@imapmail.org>]

If org-tags-alist is customized by the user, the value returned by
org-global-tags-completion-table does not include any tags from agenda
buffers and files.

This behavior contradicts the docstring of
org-global-tags-completion-table, which claims to return the list of all
tags in all agenda buffer/files.

I believe this bug was introduced with commit
4743d43dd8e448b6c440b1e4988bcd353de60cc7 in April 2016. Before that
commit, Org mode appended tags in org-tags-alist to tags gathered from
the buffer. After the commit, Org mode no longer gathers buffer tags if
org-tags-alist is defined.

Line 13731 is the key line:

        (or org-current-tag-alist (org-get-buffer-tags)))

AFAICT, org-current-tag-alist only includes tags defined in
org-tag-alist and org-tag-persistent-alist. So if these are defined, the
function will never gather the buffer tags.

As an aside, this bug makes filtering the agenda by filetags or tags in
org buffers impossible due to commit
404ac42ee51f0ac0d9cfb8fbefaefbbe96c61017, which requires a match for tag
completion when hitting "/ [TAB]" in the agenda. Since
org-global-tags-completion-table does not actually return the tags in
buffers, it is impossible to filter by them.

I can reproduce this with emacs -Q (emacs 25.3 and Org mode from git).

Best,
Matt