diff -r 29c22496dd97 -r f0def07fa82f mercurial/branchmap.py
--- a/mercurial/branchmap.py	Sun Mar 31 16:20:17 2019 +0300
+++ b/mercurial/branchmap.py	Sun Mar 31 16:27:10 2019 +0300
@@ -182,6 +182,9 @@
     def __getitem__(self, key):
         return self._entries[key]
 
+    def __contains__(self, key):
+        return key in self._entries
+
     def iteritems(self):
         return self._entries.iteritems()