bundle2: call a hook after the transaction is closed

We call a dedicated hook right after closing the transaction. This will let
people react to the transaction with all the information in hand. This hook is
experimental and will not survive in future versions.

mercurial/exchange.py

@@ -733,6 +733,8 b' def unbundle(repo, cg, heads, source, ur'
             repo.hook('b2x-pretransactionclose', throw=True, source=source,
                       url=url, pending=p, **tr.hookargs)
             tr.close()
+            repo.hook('b2x-transactionclose', source=source, url=url,
+                      **tr.hookargs)
         else:
             r = changegroup.addchangegroup(repo, cg, source, url)
     finally: