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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state