bookmark: introduce a 'applychanges' function to gather bookmark movement

We want to track bookmark movement within a transaction. For this we need a
more centralized way to update bookmarks.

For this purpose we introduce a new 'applychanges' method that apply a list of
changes encoded as '(name, node)'. We'll cover all bookmark updating code to
this new method in later changesets and add bookmark move in the transaction
when all will be migrated.

mercurial/bookmarks.py

@@ -109,6 +109,16 b' class bmstore(dict):'
         self._clean = False
         return dict.__delitem__(self, key)
 
+    def applychanges(self, repo, tr, changes):
+        """Apply a list of changes to bookmarks
+        """
+        for name, node in changes:
+            if node is None:
+                del self[name]
+            else:
+                self[name] = node
+        self.recordchange(tr)
+
     def recordchange(self, tr):
         """record that bookmarks have been changed in a transaction