windows: add a global equivalent to /etc/mercurial for *.rc processing
This follows the Unix model of processing this directory immediately after
<internal>/*.rc, and prior to the installation relative files. Since the Unix
processing supports both a directory and a file (the former overriding the
latter), and since %HOME% supports both `*.ini` and `.hgrc` (again, the former
overriding the latter), this does too. The Unix file doesn't have a `.` prefix,
so it's not used here either.
Note that this is the opposite order of processing the exe relative paths. But
since it's in agreement with Unix, %HOME% and %USERPROFILE%, it seems reasonable
to ignore that. Maybe we can change that and take a BC, because that's
something the installer should be controlling, and I can't imagine people having
both paths *and* conflicting settings.
Differential Revision: https://phab.mercurial-scm.org/D7692
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors