resolve-undo: convert struct resolve_undo_info to object_id

resolve-undo: convert struct resolve_undo_info to object_id

Convert the sha1 member of this struct to be an array of struct
object_id instead.  This change is needed to convert find_unique_abbrev.

Convert some instances of hard-coded constants to use the_hash_algo as
well.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>