templater: switch to lower-level config.parse() in _readmapfile()

I hope to modify this code to also work with resources loaded from
memory (for PyOxidizer support). For that, we'll need to handle the
lookup of relative `%include` path (not joined with the base directory
of the containing file). This patch prepares for that by using
`config.parse()` instead of `config.read()`, since the latter expects
a file in the file system.

As it happens, this change also lets us clean up the `config` class to
not need the `_includepaths` field. That will happen next.

Differential Revision: https://phab.mercurial-scm.org/D8791

mercurial/templater.py

@@ -824,7 +824,13 b' def _readmapfile(mapfile):'
 
     base = os.path.dirname(mapfile)
     conf = config.config(includepaths=[templatedir()])
-    conf.read(mapfile, remap={b'': b'templates'})
+
+    def include(rel, abs, remap, sections):
+        data = util.posixfile(abs, b'rb').read()
+        conf.parse(abs, data, sections=sections, remap=remap, include=include)
+
+    data = util.posixfile(mapfile, b'rb').read()
+    conf.parse(mapfile, data, remap={b'': b'templates'}, include=include)
 
     cache = {}
     tmap = {}