subrepo: avoid false unsafe path detection on Windows

Subrepo paths are not normalized for the OS, so what was happening in the
subsequent root path check was:

    root                  -> $TESTTMP\issue1852a\sub/repo
    util.expandpath(...)  -> $TESTTMP\issue1852a\sub/repo
    os.path.realpath(...) -> $TESTTMP\issue1852a\sub\repo

mercurial/subrepo.py

@@ -405,7 +405,7 b' class hgsubrepo(abstractsubrepo):'
         super(hgsubrepo, self).__init__(ctx, path)
         self._state = state
         r = ctx.repo()
-        root = r.wjoin(path)
+        root = r.wjoin(util.localpath(path))
         create = allowcreate and not r.wvfs.exists('%s/.hg' % path)
         # repository constructor does expand variables in path, which is
         # unsafe since subrepo path might come from untrusted source.