branchmap: introduce branchheads() method

mercurial/branchmap.py

@@ -179,6 +179,12 b' class branchcache(dict):'
     def branchtip(self, branch):
         return self._branchtip(self[branch])[0]
 
+    def branchheads(self, branch, closed=False):
+        heads = self[branch]
+        if not closed:
+            heads = [h for h in heads if h not in self._closednodes]
+        return heads
+
     def copy(self):
         """return an deep copy of the branchcache object"""
         return branchcache(self, self.tipnode, self.tiprev, self.filteredhash,