patch 9.1.0822: topline might be changed in diff mode unexpectedly

Problem:  topline might be changed in diff mode unexpectedly
          (Jaehwang Jung)
Solution: do not re-calculate topline, when using line() func
          in diff mode.

fixes: #15812
closes: #15950

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 7e117a3..a654b6d 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ