built-in stash: use the built-in `git add -p` if so configured

built-in stash: use the built-in `git add -p` if so configured

The scripted version of `git stash` called directly into the Perl script
`git-add--interactive.perl`, and this was faithfully converted to C.

However, we have a much better way to do this now: call the internal API
directly, which will now incidentally also respect the
`add.interactive.useBuiltin` setting. Let's just do this.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>