phases: define an official tuple of phases we do not share

Same motivation as for `mutablephases`, having a single definition helps with
updating phases logic.

mercurial/phases.py

@@ -127,6 +127,7 b' allphases = public, draft, secret = rang'
 trackedphases = allphases[1:]
 phasenames = ['public', 'draft', 'secret']
 mutablephases = tuple(allphases[1:])
+remotehiddenphases = tuple(allphases[2:])
 
 def _readroots(repo, phasedefaults=None):
     """Read phase roots from disk

mercurial/repoview.py

@@ -91,8 +91,8 b' def computeunserved(repo, visibilityexce'
     # fast path in simple case to avoid impact of non optimised code
     hiddens = filterrevs(repo, 'visible')
     if phases.hassecret(repo):
-        secrets = frozenset(repo._phasecache.getrevset(repo, (phases.secret,)))
-        return frozenset(hiddens | secrets)
+        secrets = repo._phasecache.getrevset(repo, phases.remotehiddenphases)
+        return frozenset(hiddens | frozenset(secrets))
     else:
         return hiddens