archival: fix a missing r'' on a kwargs check

# skip-blame just an r prefix

Differential Revision: https://phab.mercurial-scm.org/D2671

mercurial/archival.py

@@ -125,7 +125,7 b' class tarit(object):'
 
         def __init__(self, *args, **kw):
             timestamp = None
-            if 'timestamp' in kw:
+            if r'timestamp' in kw:
                 timestamp = kw.pop(r'timestamp')
             if timestamp is None:
                 self.timestamp = time.time()