tests/test-mq-qsave.t

rust-index: using `hg::index::Index` in `hg-cpython::dagops`

Hooking `headrevs` to the Rust index is straightforward as long as
we go the `PySharedRef` way. Direct attempts of obtaining a reference
to the inner `hg::index::Index` fail for lifetime reasons: the reference
is bound to the GIL, yet the `as_set` local variable is considered to
be static (the borrow checker clearly does not realize or care that this
set only stores `Revision` values).

In `rank()`, the chosen solution is the simplest as far as `hg-cpython` is
concerned, but it has the defect of removing an implementation
that would be easily adaptable if the core index did implement `RankedGraph`
(returning the same error as long as only `REVLOGV1` is supported), but that
would introduce a direct dependency of `hg-core` on the ``vcsgraph` crate.

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init repo
  $ cd repo

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state