resolve-undo: stop using `the_repository`

resolve-undo: stop using `the_repository`

Stop using `the_repository` in the "resolve-undo" subsystem by passing
in the hash algorithm when reading or writing resolve-undo information.

While we could trivially update the caller to pass in the hash algorithm
used by the index itself, we instead pass in `the_hash_algo`. This is
mostly done to stay consistent with the rest of the code in that file,
which isn't prepared to handle arbitrary repositories, either.

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