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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).