Patch text in git-add patch mode lacks whitespace highlighting

[Mike McGranahan <mike@xxxxxxxxxx>]

Hello,

I'm using version 2.25.0.windows.1. If I set config "wsErrorHighlight"
to "old", and run `git diff -p`, the resulting patch text highlights
whitespace differences in the old text.

If I then run git-add in interactive patch mode, I expect the diff to
include the whitespace highlights. But actually, it does not.

Is this a bug? Thanks for your help.

-Mike