bug#12585: diff refine should highlight space changes better (feature request)

[Juri Linkov]

>> Inline space changes can be highlighted after setting
>> `smerge-refine-ignore-whitespace' to nil
>> (maybe it should be defcustom instead of defvar?)
>
> How would I use smerge in this context?  I'm just pressing C-c C-b to
> refine a diff hunk right now.

`C-c C-b' in diff uses `smerge-refine-ignore-whitespace' from smerge.
Perhaps diff should have a duplicate option `diff-refine-ignore-whitespace'
with the same functionality but a different name prefix.