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

Change the write_object_file() function to take an "enum object_type"
instead of a "const char *type". Its callers either passed
{commit,tree,blob,tag}_type and can pass the corresponding OBJ_* type
instead, or were hardcoding strings like "blob".

This avoids the back & forth fragility where the callers of
write_object_file() would have the enum type, and convert it
themselves via type_name(). We do have to now do that conversion
ourselves before calling write_object_file_prepare(), but those
codepaths will be similarly adjusted in subsequent commits.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 4a94662..8080415 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -177,7 +177,7 @@ static void write_cached_object(struct object *obj, struct obj_buffer *obj_buf)
 	struct object_id oid;
 
 	if (write_object_file(obj_buf->buffer, obj_buf->size,
-			      type_name(obj->type), &oid) < 0)
+			      obj->type, &oid) < 0)
 		die("failed to write object %s", oid_to_hex(&obj->oid));
 	obj->flags |= FLAG_WRITTEN;
 }
@@ -243,7 +243,7 @@ static void write_object(unsigned nr, enum object_type type,
 			 void *buf, unsigned long size)
 {
 	if (!strict) {
-		if (write_object_file(buf, size, type_name(type),
+		if (write_object_file(buf, size, type,
 				      &obj_list[nr].oid) < 0)
 			die("failed to write object");
 		added_object(nr, type, buf, size);
@@ -251,7 +251,7 @@ static void write_object(unsigned nr, enum object_type type,
 		obj_list[nr].obj = NULL;
 	} else if (type == OBJ_BLOB) {
 		struct blob *blob;
-		if (write_object_file(buf, size, type_name(type),
+		if (write_object_file(buf, size, type,
 				      &obj_list[nr].oid) < 0)
 			die("failed to write object");
 		added_object(nr, type, buf, size);