patch 9.1.0743: diff mode does not handle overlapping diffs correctly

Problem:  diff mode does not handle overlapping diffs correctly
Solution: correct the logic to handle overlapping blocks
          (Yukihiro Nakadaira)

Vim merges overlapped diff blocks and it doesn't work expectedly
in some situation.

closes: #15735

Signed-off-by: Yukihiro Nakadaira <yukihiro.nakadaira@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index c391f2d..7e117a3 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ