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

mercurial/subrepoutil.py

@@ -36,7 +36,7 b' def state(ctx, ui):'
     p = config.config()
     repo = ctx.repo()
 
-    def read(rel, f, sections=None, remap=None):
+    def read(f, abs, sections=None, remap=None):
         if f in ctx:
             try:
                 data = ctx[f].data()