(vertical-motion -1) misbehaves in outline-minor-mode

[Peter Heslin]

According to the doc-string for vertical-motion, (vertical-motion 1)
should put you at the start of the next screen line and
(vertical-motion -1) should put you at the start of the previous
screen line.

It does, except that, when viewing the headings of a file in
outline-minor-mode after running hide-body, (vertical-motion -1) puts
point at the *end* of the previous screen line.

I use screen-lines.el and scroll-in-place.el, both of which use
vertical-motion to scroll, and both of which behave weirdly when
scrolling upwards in buffers where hide-body has been used.  That's
what led me to find this bug.

To reproduce it: on my month-old CVS checkout, I ran emacs -q, then
find-library to find any biggish elisp file, then M-x
outline-minor-mode and M-x hide-body.  I put point somewhere in the
middle of the folded defuns, and compared the behavior of
(vertical-motion 1) and (vertical-motion -1)

I tried to see if the same bug appeared when hiding the defun forms
with hs-hide-all, but (vertical-motion -1) seems to work just fine
with hideshow.

Peter