phases: add a repository requirement about internal phase

For internal changeset to be properly hidden, the client version needs to
support it. So we introduce a new repository requirement that will make sure
clients touching a repository that uses internal phase supports the feature.

mercurial/configitems.py

@@ -679,6 +679,9 b" coreconfigitem('format', 'usegeneraldelt"
 coreconfigitem('format', 'usestore',
     default=True,
 )
+coreconfigitem('format', 'internal-phase',
+    default=False,
+)
 coreconfigitem('fsmonitor', 'warn_when_unused',
     default=True,
 )

mercurial/localrepo.py

@@ -394,6 +394,7 b' class localrepository(object):'
         'relshared',
         'dotencode',
         'exp-sparse',
+        'internal-phase'
     }
     openerreqs = {
         'revlogv1',
@@ -2427,5 +2428,8 b' def newreporequirements(repo):'
         # generaldelta is implied by revlogv2.
         requirements.discard('generaldelta')
         requirements.add(REVLOGV2_REQUIREMENT)
+    # experimental config: format.internal-phase
+    if repo.ui.configbool('format', 'internal-phase'):
+        requirements.add('internal-phase')
 
     return requirements