phases: define an official tuple of mutable phases

Such tuple was already manually defined in a couple of place. Having an
official definition makes it easy to introduce of new phases.

mercurial/obsolete.py

@@ -853,7 +853,7 b' def clearobscaches(repo):'
 
 def _mutablerevs(repo):
     """the set of mutable revision in the repository"""
-    return repo._phasecache.getrevset(repo, (phases.draft, phases.secret))
+    return repo._phasecache.getrevset(repo, phases.mutablephases)
 
 @cachefor('obsolete')
 def _computeobsoleteset(repo):

mercurial/phases.py

@@ -126,6 +126,7 b' from . import ('
 allphases = public, draft, secret = range(3)
 trackedphases = allphases[1:]
 phasenames = ['public', 'draft', 'secret']
+mutablephases = tuple(allphases[1:])
 
 def _readroots(repo, phasedefaults=None):
     """Read phase roots from disk

mercurial/repoview.py

@@ -77,8 +77,7 b' def computehidden(repo, visibilityexcept'
         if visibilityexceptions:
             hidden -= visibilityexceptions
         pfunc = repo.changelog.parentrevs
-        mutablephases = (phases.draft, phases.secret)
-        mutable = repo._phasecache.getrevset(repo, mutablephases)
+        mutable = repo._phasecache.getrevset(repo, phases.mutablephases)
 
         visible = mutable - hidden
         _revealancestors(pfunc, hidden, visible)