["BUG"] builtin add-interactive does not honor 'color.diff. frag'

[Philippe Blain <levraiphilippeblain@xxxxxxxxx>]

Hi Johannes,

I tried the builtin add-interactive for the first time today, 
and noticed a very minor difference with the Perl version.
The title says it all.

Repro:

# Change the default color
$ git config color.diff.frag "magenta bold"
# Compare
$ git add -p
# and
$ git -c add.interactive.useBuiltin=true add -p 
# I'm on latest master
$ git --version
git version 2.29.2.154.g7f7ebe054a

Cheers,
Philippe.