applybundle: take source as argument

We allow specifying the source to carry it to hooks. This gets us closer to
'bundle1.apply(...)' and will allow us to remove regressions in multiple places
where we forget to pass the source to hooks.

mercurial/bundle2.py

@@ -302,9 +302,11 b' def _notransaction():'
     to be created"""
     raise TransactionUnavailable()
 
-def applybundle(repo, unbundler, tr, op=None):
+def applybundle(repo, unbundler, tr, source=None, op=None):
     # transform me into unbundler.apply() as soon as the freeze is lifted
     tr.hookargs['bundle2'] = '1'
+    if source is not None and 'source' not in tr.hookargs:
+        tr.hookargs['source'] = source
     return processbundle(repo, unbundler, lambda: tr, op=op)
 
 def processbundle(repo, unbundler, transactiongetter=None, op=None):