subrepoutil: use relative path for looking up config `%include`s The code was already working with relative paths in practice, since it passed in a (repo-)relative path into its local `read()` function. So all that this patch actually does is to switch to rename variables so we use the same path by a different name. This gets us closer to removing the "absolute" path from the `include` callback. Differential Revision: https://phab.mercurial-scm.org/D8794