object-file API: have hash_object_file() take "enum object_type"

Change the hash_object_file() function to take an "enum
object_type".

Since a preceding commit all of its callers are passing either
"{commit,tree,blob,tag}_type", or the result of a call to type_name(),
the parse_object() caller that would pass NULL is now using
stream_object_signature().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-check.c

@@ -143,7 +143,7 @@ static int verify_packfile(struct repository *r,
 				    oid_to_hex(&oid), p->pack_name,
 				    (uintmax_t)entries[i].offset);
 		else if (data && check_object_signature(r, &oid, data, size,
-							type_name(type)) < 0)
+							type) < 0)
 			err = error("packed %s from %s is corrupt",
 				    oid_to_hex(&oid), p->pack_name);
 		else if (!data && stream_object_signature(r, &oid) < 0)