add-patch: split out `struct interactive_options`

add-patch: split out `struct interactive_options`

The `struct add_p_opt` is reused both by our infra for "git add -p" and
"git add -i". Users of `run_add_i()` for example are expected to pass
`struct add_p_opt`. This is somewhat confusing and raises the question
of which options apply to what part of the stack.

But things are even more confusing than that: while callers are expected
to pass in `struct add_p_opt`, these options ultimately get used to
initialize a `struct add_i_state` that is used by both subsystems. So we
are basically going full circle here.

Refactor the code and split out a new `struct interactive_options` that
hosts common options used by both. These options are then applied to a
`struct interactive_config` that hosts common configuration.

This refactoring doesn't yet fully detangle the two subsystems from one
another, as we still end up calling `init_add_i_state()` in the "git add
-p" subsystem. This will be fixed in a subsequent commit.

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