workingctx: overload bookmarks() to return parents' bookmarks

This implements workingctx.bookmarks() parallel to workingctx.tags(),
overloading the implementation from changectx.

mercurial/context.py

@@ -730,6 +730,12 b' class workingctx(changectx):'
             t.extend(p.tags())
         return t
 
+    def bookmarks(self):
+        b = []
+        for p in self.parents():
+            b.extend(p.bookmarks())
+        return b
+
     def children(self):
         return []