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.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}