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>
diff --git a/diffcore-rename.c b/diffcore-rename.c
index bebd4ed..c0422d9 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -261,7 +261,7 @@ static unsigned int hash_filespec(struct repository *r,
 		if (diff_populate_filespec(r, filespec, NULL))
 			return 0;
 		hash_object_file(r->hash_algo, filespec->data, filespec->size,
-				 "blob", &filespec->oid);
+				 OBJ_BLOB, &filespec->oid);
 	}
 	return oidhash(&filespec->oid);
 }