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

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