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>

dir.c

@@ -1113,7 +1113,7 @@ static int add_patterns(const char *fname, const char *base, int baselen,
 				       &istate->cache[pos]->oid);
 			else
 				hash_object_file(the_hash_algo, buf, size,
-						 "blob", &oid_stat->oid);
+						 OBJ_BLOB, &oid_stat->oid);
 			fill_stat_data(&oid_stat->stat, &st);
 			oid_stat->valid = 1;
 		}