add -i: use the built-in version when feature.experimental is set

add -i: use the built-in version when feature.experimental is set

We have had parallel implementations of "add -i/-p" since 2.25 and
have been using them from various codepaths since 2.26 days, but
never made the built-in version the default.

We have found and fixed a handful of corner case bugs in the
built-in version, and it may be a good time to start switching over
the user base from the scripted version to the built-in version.
Let's enable the built-in version for those who opt into the
feature.experimental guinea-pig program to give wider exposure.

Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>