bundle2: grab kwarg using sysstr

# skip-blame just an r prefix on a string

Differential Revision: https://phab.mercurial-scm.org/D4691

mercurial/bundle2.py

@@ -1672,7 +1672,7 b' def _formatrequirementsparams(requiremen'
     return params
 
 def addpartbundlestream2(bundler, repo, **kwargs):
-    if not kwargs.get('stream', False):
+    if not kwargs.get(r'stream', False):
         return
 
     if not streamclone.allowservergeneration(repo):