branchmap: implement __contains__()

We have good occurences of `if branch in branchmap()` in our code. If
__contains__() is not implemented then it will use __iter__() to find whether
the element exists or not which is not good.

I am bit confused that whether I should move existing callers to hasbranch() or
this patch is a good way.

Differential Revision: https://phab.mercurial-scm.org/D6206

mercurial/branchmap.py

@@ -182,6 +182,9 b' class branchcache(object):'
     def __getitem__(self, key):
         return self._entries[key]
 
+    def __contains__(self, key):
+        return key in self._entries
+
     def iteritems(self):
         return self._entries.iteritems()