tests/test-debugrename.t

rust-cpython: rename PyLeakedRef to PyLeaked

This series will make PyLeaked* behave more like a Python iterator, which
means mutation of the owner object will be allowed and the leaked reference
(i.e. the iterator) will be invalidated instead.

I'll add PyLeakedRef/PyLeakedRefMut structs which will represent a "borrowed"
state, and prevent the underlying value from being mutably borrowed while the
leaked reference is in use:

  let shared = self.inner_shared(py);
  let leaked = shared.leak_immutable();
  {
      let leaked_ref: PyLeakedRef<_> = leaked.borrow(py);
      shared.borrow_mut();  // panics since the underlying value is borrowed
  }
  shared.borrow_mut();  // allowed

The relation between PyLeaked* structs is quite similar to RefCell/Ref/RefMut,
but the implementation can't be reused because the borrowing state will have
to be shared across objects having no lifetime relation.

PyLeaked isn't named as PyLeakedCell since it isn't actually a cell in that
leaked.borrow_mut() will require &mut self.

  $ hg init
  $ 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