ancestor: incrementalmissingancestors.basesheads()

This new method will avoid the need to access the `bases` attribute directly
in `setdiscovery`, and to prefilter `nullrev` before passing it to the
`heads()` revset.

Being a method, it can transparently be reimplemented in a Rust (or any native)
version.

Differential Revision: https://phab.mercurial-scm.org/D5582

mercurial/ancestor.py

@@ -11,6 +11,7 b' import heapq'
 
 from .node import nullrev
 from . import (
+    dagop,
     policy,
     pycompat,
 )
@@ -162,6 +163,9 b' class incrementalmissingancestors(object'
         '''grow the ancestor set by adding new bases'''
         self.bases.update(newbases)
 
+    def basesheads(self):
+        return dagop.headrevs(self.bases, self.pfunc)
+
     def removeancestorsfrom(self, revs):
         '''remove all ancestors of bases from the set revs (in place)'''
         bases = self.bases