repo: Rename "stage" to "write" in the API

An earlier version of this API acted like git in that some objects
would be staged in a temporary directory which would be then committed
in one go by moving files around. The API doesn't match most users
expectations though, as while the stage is nice as a high-level API
it isn't really suited for low-level APIs.

While the stage was removed, the APIs were never renamed. Rename
them now so that they match expectations.

https://bugzilla.gnome.org/show_bug.cgi?id=707644

src/libostree/ostree-repo.c

@@ -54,12 +54,12 @@
  * To store content in the repo, first start a transaction with
  * ostree_repo_prepare_transaction().  Then create a
  * #OstreeMutableTree, and apply functions such as
- * ostree_repo_stage_directory_to_mtree() to traverse a physical
- * filesystem and stage content, possibly multiple times.
+ * ostree_repo_write_directory_to_mtree() to traverse a physical
+ * filesystem and write content, possibly multiple times.
  * 
- * Once the #OstreeMutableTree is complete, stage all of its metadata
- * with ostree_repo_stage_mtree(), and finally create a commit with
- * ostree_repo_stage_commit().
+ * Once the #OstreeMutableTree is complete, write all of its metadata
+ * with ostree_repo_write_mtree(), and finally create a commit with
+ * ostree_repo_write_commit().
  */
 typedef struct {
   GObjectClass parent_class;