patch 9.0.1213: adding a line below the last one does not expand fold

Problem:    Adding a line below the last one does not expand fold.
Solution:   Do not skip mark_adjust() when adding lines below the last one.
            (Brandon Simmons, closes #11832, closes #10698)

src/testdir/test_fold.vim

@@ -1683,4 +1683,17 @@ func Test_indent_with_L_command()
 V{zf8=Lu
   bwipe!
 endfunc
+
+" Make sure that when there is a fold at the bottom of the buffer and a newline
+" character is appended to the line, the fold gets expanded (instead of the new
+" line not being part of the fold).
+func Test_expand_fold_at_bottom_of_buffer()
+  new
+  " create a fold on the only line
+  fold
+  execute "normal A\<CR>"
+  call assert_equal([1, 1], range(1, 2)->map('foldlevel(v:val)'))
+
+  bwipe!
+endfunc