patch 8.2.3394: filler lines are wrong when changing text in diff mode

Problem:    Filler lines are wrong when changing text in diff mode.
Solution:   Don't change the filler lines on every change.  Check
            scrollbinding when updating the filler lines. (closes #8809)
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 5400de6..268c3e9 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -993,6 +993,30 @@
   call delete('XdiffSetup')
 endfunc
 
+func Test_diff_with_scroll_and_change()
+  CheckScreendump
+
+  let lines =<< trim END
+	call setline(1, range(1, 15))
+	vnew
+	call setline(1, range(9, 15))
+	windo diffthis
+	wincmd h
+	exe "normal Gl5\<C-E>"
+  END
+  call writefile(lines, 'Xtest_scroll_change')
+  let buf = RunVimInTerminal('-S Xtest_scroll_change', {})
+
+  call VerifyScreenDump(buf, 'Test_diff_scroll_change_01', {})
+
+  call term_sendkeys(buf, "ax\<Esc>")
+  call VerifyScreenDump(buf, 'Test_diff_scroll_change_02', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xtest_scroll_change')
+endfunc
+
 func Test_diff_with_cursorline()
   CheckScreendump