vim-patch:9.1.1753: defaults: 'diffopt' option value can be improved (#35727)

Problem:  defaults: 'diffopt' option value can be improved
Solution: Update diffopt defaults to include "indent-heuristic" and
          "inline:char" (Yee Cheng Chin)

The default diff options have not been updated much despite new
functionality having been added to Vim.

- indent-heurstic: This has been enabled by default in Git since
  33de716387 in 2017. Given that Vim uses xdiff from Git, it makes sense
  to track the default configuration from Git.

- inline:char: This turns on character-wise inline highlighting which is
  generally much better than the default inline:simple. It has been
  implemented since vim/vim#16881 and we have not seen reports of any issues
  with it, and it has received good feedbacks.

closes: vim/vim#18255

976b365305

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

test/functional/ui/diff_spec.lua

@@ -1249,7 +1249,7 @@ end)
 it('diff updates line numbers below filler lines', function()
   local screen = Screen.new(40, 14)
   exec([[
-    set diffopt=internal,filler,closeoff
+    set diffopt=internal,filler
     call setline(1, ['a', 'a', 'a', 'y', 'b', 'b', 'b', 'b', 'b'])
     vnew
     call setline(1, ['a', 'a', 'a', 'x', 'x', 'x', 'b', 'b', 'b', 'b', 'b'])