pathutil: use `finddirs_rev_noroot` instead of `parts`
The benefit this brings is very tiny, if it's even there,
since we still didn't get rid of the [parts] computation.
It probably won't be worth it without the subsequent patch
that adds one more use of [finddirs_rev_noroot]
#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