tests/test-debugrename.t

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]

  $ hg init repo
  $ cd repo
  $ echo a > a
  $ hg ci -Am t
  adding a

  $ hg mv a b
  $ hg ci -Am t1
  $ hg debugrename b
  b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3

  $ hg mv b a
  $ hg ci -Am t2
  $ hg debugrename a
  a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980

  $ hg debugrename --rev 1 b
  b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3