tests/test-xdg.t

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

#if no-windows no-osx

  $ mkdir -p xdgconf/hg
  $ echo '[ui]' > xdgconf/hg/hgrc
  $ echo 'username = foobar' >> xdgconf/hg/hgrc
  $ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
  $ unset HGRCPATH
  $ hg config ui.username 2>/dev/null
  foobar

#endif