add -i (built-in): use correct names to load color.diff.* config

add -i (built-in): use correct names to load color.diff.* config

The builtin version of add-interactive mistakenly loads diff colors from
color.interactive.* instead of color.diff.*. It also accidentally spells
`frag` as `fraginfo`.

Let's fix that.

Note also that we don't respect the historical `diff.color.*`. The perl
version never did, and those have been deprecated since 2007.

Reported-by: Philippe Blain <redacted>
Co-authored-by: Jeff King <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>