patch 9.1.0526: Unwanted cursor movement with pagescroll at start of buffer

Problem:  Cursor is moved to bottom of window trying to pagescroll when
          already at the start of the buffer (Asheq Imran, after v9.1.0357)
Solution: Don't move cursor when buffer content did not move.
          (Luuk van Baal)

closes: #15139

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 83594d2..398bf29 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -4257,6 +4257,9 @@
   call assert_equal(18, line('.'))
   exe "norm! \<C-B>\<C-F>"
   call assert_equal(9, line('.'))
+  " Not when already at the start of the buffer.
+  exe "norm! ggj\<C-B>"
+  call assert_equal(2, line('.'))
   bwipe!
 endfunc