context: use native string when peeking in __dict__

# skip-blame because we're just adding a prefix on a string constant

Differential Revision: https://phab.mercurial-scm.org/D1893

mercurial/context.py

@@ -1051,7 +1051,7 b' class basefilectx(object):'
             # renamed filectx won't have a filelog yet, so set it
             # from the cache to save time
             for p in pl:
-                if not '_filelog' in p.__dict__:
+                if not r'_filelog' in p.__dict__:
                     p._filelog = getlog(p.path())
 
             return pl