add-patch: remove dependency on "add-interactive" subsystem

add-patch: remove dependency on "add-interactive" subsystem

With the preceding commit we have split out interactive configuration
that is used by both "git add -p" and "git add -i". But we still
initialize that configuration in the "add -p" subsystem by calling
`init_add_i_state()`, even though we only do so to initialize the
interactive configuration as well as a repository pointer.

Stop doing so and instead store and initialize the interactive
configuration in `struct add_p_state` directly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>