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>

src/nvim/options.lua

@@ -2374,7 +2374,7 @@ local options = {
     {
       abbreviation = 'dip',
       cb = 'did_set_diffopt',
-      defaults = 'internal,filler,closeoff,inline:simple,linematch:40',
+      defaults = 'internal,filler,closeoff,indent-heuristic,inline:char,linematch:40',
       -- Keep this in sync with diffopt_changed().
       values = {
         'filler',