bundle2: make source a mandatory argument for bundle2.applybundle() (API)

Currently all the callers in the core pass the source argument, making it
mandatory will help us storing right source value in bundleoperation() class.

Differential Revision: https://phab.mercurial-scm.org/D2996

mercurial/bundle2.py

@@ -342,7 +342,7 b' def _notransaction():'
     to be created"""
     raise TransactionUnavailable()
 
-def applybundle(repo, unbundler, tr, source=None, url=None, **kwargs):
+def applybundle(repo, unbundler, tr, source, url=None, **kwargs):
     # transform me into unbundler.apply() as soon as the freeze is lifted
     if isinstance(unbundler, unbundle20):
         tr.hookargs['bundle2'] = '1'