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.

mercurial/commands.py

@@ -4030,7 +4030,7 b' def phase(ui, repo, *revs, **opts):'
     # search for a unique phase argument
     targetphase = None
     for idx, name in enumerate(phases.phasenames):
-        if opts[name]:
+        if opts.get(name, False):
             if targetphase is not None:
                 raise error.Abort(_('only one phase can be specified'))
             targetphase = idx