exchange: replace a "not x in ys" by more Pythonic "x not in ys"

Found by one of our (Google-)internal tools.

Differential Revision: https://phab.mercurial-scm.org/D7546

mercurial/exchange.py

@@ -2577,7 +2577,7 b' def _getbundlephasespart('
 ):
     """add phase heads part to the requested bundle"""
     if kwargs.get('phases', False):
-        if not b2caps or not b'heads' in b2caps.get(b'phases'):
+        if not b2caps or b'heads' not in b2caps.get(b'phases'):
             raise error.Abort(_(b'no common phases exchange method'))
         if heads is None:
             heads = repo.heads()