revlog: add a callback "tracking" duplicate node addition

If a changegroup contains node already added to the repository, they will be
skipped. Skipping them is the right behavior (we don't need to store things
twice), but it can hide some information to the code doing the unbundle (eg:
shelve looking for the tip of the bundle).

The first step to improve this situation is to add a low level callback. We do
not need this tracking on all revlog, so actual tracking will be added in the
next changeset.

mercurial/revlog.py

@@ -1745,6 +1745,10 b' class revlog(object):'
         tr.replace(self.indexfile, trindex * self._io.size)
         self._chunkclear()
 
+    def _nodeduplicatecallback(self, transaction, node):
+        """called when trying to add a node already stored.
+        """
+
     def addrevision(self, text, transaction, link, p1, p2, cachedelta=None,
                     node=None, flags=REVIDX_DEFAULT_FLAGS, deltacomputer=None):
         """add a revision to the log
@@ -2026,6 +2030,7 b' class revlog(object):'
                 nodes.append(node)
 
                 if node in self.nodemap:
+                    self._nodeduplicatecallback(transaction, node)
                     # this can happen if two branches make the same change
                     continue