fix(scrollbind): properly take filler/virtual lines into account

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

src/nvim/diff.c

@@ -2143,7 +2143,11 @@ int diff_check_with_linestatus(win_T *wp, linenr_T lnum, int *linestatus)
     return 0;
   }
 
-  if (!dp->is_linematched && diff_linematch(dp)) {
+  // Don't run linematch when lnum is offscreen.
+  // Useful for scrollbind calculations which need to count all the filler lines
+  // above the screen.
+  if (lnum >= wp->w_topline && lnum < wp->w_botline
+      && !dp->is_linematched && diff_linematch(dp)) {
     run_linematch_algorithm(dp);
   }