add-patch: split out header from "add-interactive.h"

add-patch: split out header from "add-interactive.h"

While we have a "add-patch.c" code file, its declarations are part of
"add-interactive.h". This makes it somewhat harder than necessary to
find relevant code and to identify clear boundaries between the two
subsystems.

Split up concerns and move declarations that relate to "add-patch.c"
into a new "add-patch.h" header.

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