ancestor: add a way to test whether a missing ancestor object has bases

This is pretty trivial so there's no unit test coverage for it.

This will be used by setdiscovery.

mercurial/ancestor.py

@@ -146,6 +146,10 b' class incrementalmissingancestors(object'
             self.bases.add(nullrev)
         self.pfunc = pfunc
 
+    def hasbases(self):
+        '''whether the common set has any non-trivial bases'''
+        return self.bases and self.bases != set([nullrev])
+
     def missingancestors(self, revs):
         '''return all the ancestors of revs that are not ancestors of self.bases