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/cache-tree.c b/cache-tree.c
index fdbbd19..6752f69 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -432,14 +432,14 @@ static int update_one(struct cache_tree *it,
 	if (repair) {
 		struct object_id oid;
 		hash_object_file(the_hash_algo, buffer.buf, buffer.len,
-				 tree_type, &oid);
+				 OBJ_TREE, &oid);
 		if (has_object_file_with_flags(&oid, OBJECT_INFO_SKIP_FETCH_OBJECT))
 			oidcpy(&it->oid, &oid);
 		else
 			to_invalidate = 1;
 	} else if (dryrun) {
 		hash_object_file(the_hash_algo, buffer.buf, buffer.len,
-				 tree_type, &it->oid);
+				 OBJ_TREE, &it->oid);
 	} else if (write_object_file_flags(buffer.buf, buffer.len, OBJ_TREE,
 					   &it->oid, flags & WRITE_TREE_SILENT
 					   ? HASH_SILENT : 0)) {
@@ -948,7 +948,7 @@ static int verify_one(struct repository *r,
 		strbuf_addf(&tree_buf, "%o %.*s%c", mode, entlen, name, '\0');
 		strbuf_add(&tree_buf, oid->hash, r->hash_algo->rawsz);
 	}
-	hash_object_file(r->hash_algo, tree_buf.buf, tree_buf.len, tree_type,
+	hash_object_file(r->hash_algo, tree_buf.buf, tree_buf.len, OBJ_TREE,
 			 &new_oid);
 	if (!oideq(&new_oid, &it->oid))
 		BUG("cache-tree for path %.*s does not match. "