Introduce entry point add_interactive and add_files_to_cache

Introduce entry point add_interactive and add_files_to_cache

This refactors builtin-add.c a little to provide a unique entry point
for launching git add --interactive, which will be used by
builtin-commit too.  If we later want to make add --interactive a
builtin or change how it is launched, we just start from this function.

It also exports the private function update() which is used to
add all modified paths to the index as add_files_to_cache().

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>