phases: extract the core of boundary retraction in '_retractboundary'

At the moment the 'retractboundary' function is called for multiple reasons:

First, actually retracting boundaries. There are only two cases for theses:
'hg phase --force' and 'hg qimport'. This will need extra graph computation to
retrieve the phase changes.

Second, setting the phases of newly added changesets. In this case we already
know all the affected nodes and we just needs to register different
information (old phase is None).

Third, when reducing the set of roots when advancing phase. The phase are
already properly tracked so we do not needs anything else in this case.

To deal with this difference in phase tracking, we extract the core logic into
a private method that all three cases can use.

mercurial/phases.py

@@ -331,10 +331,14 @@ class phasecache(object):
                 delroots.extend(olds - roots)
         # declare deleted root in the target phase
         if targetphase != 0:
-            self.retractboundary(repo, tr, targetphase, delroots)
+            self._retractboundary(repo, tr, targetphase, delroots)
         repo.invalidatevolatilesets()
 
     def retractboundary(self, repo, tr, targetphase, nodes):
+        self._retractboundary(repo, tr, targetphase, nodes)
+        repo.invalidatevolatilesets()
+
+    def _retractboundary(self, repo, tr, targetphase, nodes):
         # Be careful to preserve shallow-copied values: do not update
         # phaseroots values, replace them.
 
@@ -343,6 +347,7 @@ class phasecache(object):
         newroots = [n for n in nodes
                     if self.phase(repo, repo[n].rev()) < targetphase]
         if newroots:
+
             if nullid in newroots:
                 raise error.Abort(_('cannot change null revision phase'))
             currentroots = currentroots.copy()
@@ -360,7 +365,6 @@ class phasecache(object):
             finalroots.update(ctx.node() for ctx in updatedroots)
 
             self._updateroots(targetphase, finalroots, tr)
-        repo.invalidatevolatilesets()
 
     def filterunknown(self, repo):
         """remove unknown nodes from the phase boundary