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-private.h

@@ -90,7 +90,7 @@ _ostree_repo_get_object_path (OstreeRepo   *self,
                               OstreeObjectType type);
 
 gboolean
-_ostree_repo_stage_directory_meta (OstreeRepo   *self,
+_ostree_repo_write_directory_meta (OstreeRepo   *self,
                                    GFileInfo    *file_info,
                                    GVariant     *xattrs,
                                    guchar      **out_csum,