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.

phase: handle phase with no command flag Before this changeset, all existing phases have a corresponding flag on the `hg phase` command. (eg: `hg phase --draft`). After this changeset, a phase can exists without having an associated flag. This is useful to introduce a new internal phase that we do not want to expose to user.