patch 9.1.1753: defaults: 'diffopt' option value can be improved

patch 9.1.1753: defaults: 'diffopt' option value can be improved

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 #16881 and we have not seen reports of any issues
  with it, and it has received good feedbacks.

closes: #18255

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>