phases: track phase changes from 'retractboundary'

We adds new computation to find and record the revision affected by the
boundary retraction. This add more complication to the function but this seems
fine since it is only used in a couple of rare and explicit cases (`hg phase
--force` and `hg qimport`).

Having strong tracking of phase changes is worth the effort.

mercurial/phases.py

@@ -348,7 +348,30 b' class phasecache(object):'
         repo.invalidatevolatilesets()
 
     def retractboundary(self, repo, tr, targetphase, nodes):
-        self._retractboundary(repo, tr, targetphase, nodes)
+        oldroots = self.phaseroots[:targetphase + 1]
+        if tr is None:
+            phasetracking = None
+        else:
+            phasetracking = tr.changes.get('phases')
+        repo = repo.unfiltered()
+        if (self._retractboundary(repo, tr, targetphase, nodes)
+            and phasetracking is not None):
+
+            # find the affected revisions
+            new = self.phaseroots[targetphase]
+            old = oldroots[targetphase]
+            affected = set(repo.revs('(%ln::) - (%ln::)', new, old))
+
+            # find the phase of the affected revision
+            for phase in xrange(targetphase, -1, -1):
+                if phase:
+                    roots = oldroots[phase]
+                    revs = set(repo.revs('%ln::%ld', roots, affected))
+                    affected -= revs
+                else: # public phase
+                    revs = affected
+                for r in revs:
+                    _trackphasechange(phasetracking, r, phase, targetphase)
         repo.invalidatevolatilesets()
 
     def _retractboundary(self, repo, tr, targetphase, nodes):