sequencer.c: remove implicit dependency on the_repository

sequencer.c: remove implicit dependency on the_repository

Note that the_hash_algo stays, even if we can easily replace it with
repo->hash_algo. My reason is I still believe tying hash_algo to a
struct repository is a wrong move. But if I'm wrong, we can always go
for another round of conversion.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>