Problem:
`'scrollbind'` does not work properly if the window being scrolled
automatically contains any filler/virtual lines (except for diff filler
lines).
This is because when the scrollbind check is done, the logic only
considers changes to topline which are represented as line numbers.
Solution:
Write the logic for determine the scroll amount to take into account
filler/virtual lines.
Fixes#29751
573a71469d