phase: use `_phase` revset instead of string interpolation The previous code was hackyer and assume all phases had an associated revset. The later might no longer be true once we introduce more internal phase.

phase: expose a `_phase(idx)` revset Internally phase related revset are calling the `_phase` function. We expose it as an internal revset. This is useful to refer to phase in revset doing debatable things around the phase name.