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 <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/ls-files.c

@@ -271,7 +271,7 @@ static void show_ru_info(const struct index_state *istate)
 			if (!ui->mode[i])
 				continue;
 			printf("%s%06o %s %d\t", tag_resolve_undo, ui->mode[i],
-			       find_unique_abbrev(ui->sha1[i], abbrev),
+			       find_unique_abbrev(ui->oid[i].hash, abbrev),
 			       i + 1);
 			write_name(path);
 		}