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.
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors