discovery: ensure that missingheads are always heads of everything we tried

This only alter the third block executed when:
    * repo have secret changeset
    * onlyheads is not None

In other case findcommonoutgoing already had this behavior

mercurial/discovery.py

@@ -124,8 +124,7 b' def findcommonoutgoing(repo, other, only'
                 missing.append(node)
         if excluded:
             # update missing heads
-            rset = repo.set('heads(%ln)', missing)
-            missingheads = [ctx.node() for ctx in rset]
+            missingheads = phases.newheads(repo, onlyheads, excluded)
         else:
             missingheads = onlyheads
         og.missingheads = missingheads