applybundle: set 'bundle2=1' env for all transaction

This should be set for all bundle2 application, we enforce that at a low level.
This is for courtesy with hooks.

mercurial/bundle2.py

@@ -304,6 +304,7 b' def _notransaction():'
 
 def applybundle(repo, unbundler, tr, op=None):
     # transform me into unbundler.apply() as soon as the freeze is lifted
+    tr.hookargs['bundle2'] = '1'
     return processbundle(repo, unbundler, lambda: tr, op=op)
 
 def processbundle(repo, unbundler, transactiongetter=None, op=None):