subrepoutil: use relative path for looking up config %includes
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