phases: mechanism to allow extension to alter initial computation of phase

This commit add a whennodata list where extension can register a callback to be
called if no phase related data are found in the repository.

The goal is to ensure the existing extension that move phase data in 2.1 can
compute consistent phase boundary for existing repo.

mercurial/localrepo.py

@@ -37,6 +37,10 @@ class localrepository(repo.repository):
         self.baseui = baseui
         self.ui = baseui.copy()
         self._dirtyphases = False
+        # A list of callback to shape the phase if no data were found.
+        # Callback are in the form: func(repo, roots) --> processed root.
+        # This list it to be filled by extension during repo setup
+        self._phasedefaults = []
 
         try:
             self.ui.readconfig(self.join("hgrc"), self.root)

mercurial/phases.py

@@ -109,7 +109,6 @@ phasenames = ['public', 'draft', 'secret']
 def readroots(repo):
     """Read phase roots from disk"""
     roots = [set() for i in allphases]
-    roots[0].add(nullid)
     try:
         f = repo.sopener('phaseroots')
         try:
@@ -121,6 +120,8 @@ def readroots(repo):
     except IOError, inst:
         if inst.errno != errno.ENOENT:
             raise
+        for f in repo._phasedefaults:
+            roots = f(repo, roots)
     return roots
 
 def writeroots(repo):

mercurial/statichttprepo.py

@@ -86,6 +86,7 @@ class statichttprepository(localrepo.localrepository):
 
         opener = build_opener(ui, authinfo)
         self.opener = opener(self.path)
+        self._phasedefaults = []
 
         try:
             requirements = scmutil.readrequires(self.opener, self.supported)