* lisp/outline.el: Improve new feature of preserving outlines after revert.

* lisp/outline.el: Improve new feature of preserving outlines after revert.

(outline-minor-mode): Move adding hook 'outline-revert-buffer-rehighlight'
to the same code branch that calls 'outline-minor-mode-highlight-buffer'.
(outline-revert-buffer-rehighlight): Remove same conditions
already existing in 'outline-minor-mode'.
(outline-hidden-headings-regexp): Remove function.
(outline-hidden-headings-paths)
(outline-hidden-headings-restore-paths): New functions
that save and restore complete paths instead of flat regexps.
(outline-revert-buffer-restore-visibility): Use
'outline-hidden-headings-paths' and
'outline-hidden-headings-restore-paths'.