org-colview.el: Fix several bugs

org-colview.el: Fix several bugs

* org-colview.el (org-columns-goto-top-level): Correctly move
the marker `org-columns-top-level-marker'.
(org-agenda-columns): Don't set
`org-agenda-overriding-columns-format' as a buffer variable,
as we only need it dynamically.
(org-agenda-colview-summarize): Fix a bug in returning the
match string.

(The problem with `org-columns-goto-top-level' was introduced
by the commit 8d9e83.)

Thanks to Hannes Schulz and Jacob Riko who reported this.