rerere.c: remove implicit dependency on the_index

rerere.c: remove implicit dependency on the_index

The reason rerere(), rerere_forget() and rerere_remaining() take a
struct repository instead of struct index_state is not obvious from
the patch:

Deep in update_paths() and find_conflict(), hold_locked_index() and
read_index() are called. These functions assumes the index path at
$GIT_DIR/index which is not always true when you take an arbitrary
index state. Taking a repository will allow us to point to the right
index path later when we replace them with repo_ versions.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>