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]

mercurial/pathutil.py

@@ -107,14 +107,12 b' class pathauditor:'
                     )
 
         if self._realfs:
-            parts.pop()
             # It's important that we check the path parts starting from the root.
             # We don't want to add "foo/bar/baz" to auditeddir before checking if
             # there's a "foo/.hg" directory. This also means we won't accidentally
             # traverse a symlink into some other filesystem (which is potentially
             # expensive to access).
-            for i in range(len(parts)):
-                prefix = pycompat.ossep.join(parts[: i + 1])
+            for prefix in finddirs_rev_noroot(path):
                 if prefix in self.auditeddir:
                     res = self.auditeddir[prefix]
                 else: