bundle2: call a hook prior to closing the transaction

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

mercurial/exchange.py

@@ -728,6 +728,10 b' def unbundle(repo, cg, heads, source, ur'
             tr = repo.transaction('unbundle')
             tr.hookargs['bundle2-exp'] = '1'
             r = bundle2.processbundle(repo, cg, lambda: tr).reply
+            cl = repo.unfiltered().changelog
+            p = cl.writepending() and repo.root or ""
+            repo.hook('b2x-pretransactionclose', throw=True, source=source,
+                      url=url, pending=p, **tr.hookargs)
             tr.close()
         else:
             r = changegroup.addchangegroup(repo, cg, source, url)