vim-patch:8.2.1058: multiline conceal causes display errors

Problem:    Multiline conceal causes display errors.
Solution:   Do not allow conceal cross over EOL. (closes vim/vim#6326, closes vim/vim#4854,
            closes vim/vim#6302)
fc838d6cb0

Port test_conceal.vim but skip tests that require screendumps.

src/nvim/testdir/test_diffmode.vim

@@ -1007,6 +1007,18 @@ func Test_diff_rnu()
   call delete('Xtest_diff_rnu')
 endfunc
 
+func Test_diff_multilineconceal()
+  new
+  diffthis
+
+  new
+  call matchadd('Conceal', 'a\nb', 9, -1, {'conceal': 'Y'})
+  set cole=2 cocu=n
+  call setline(1, ["a", "b"])
+  diffthis
+  redraw
+endfunc
+
 func Test_diff_and_scroll()
   " this was causing an ml_get error
   set ls=2