ancestor.missingancestors: calculate start point after filtering revsvisit

Any revs that are filtered out are also in basesvisit, which means they
wouldn't be returned in the missing list anyway. There's no need to explore
such revs or their ancestors.

The 'if not revsvisit' check moves down because we can't call max() on an empty
set.

mercurial/ancestor.py

@@ -148,13 +148,13 b' def missingancestors(revs, bases, pfunc)'
 
     revsvisit = set(revs)
     basesvisit = set(bases)
-    if not revsvisit:
-        return []
     if not basesvisit:
         basesvisit.add(nullrev)
-    start = max(max(revsvisit), max(basesvisit))
     bothvisit = revsvisit.intersection(basesvisit)
     revsvisit.difference_update(bothvisit)
+    if not revsvisit:
+        return []
+    start = max(max(revsvisit), max(basesvisit))
     # At this point, we hold the invariants that:
     # - revsvisit is the set of nodes we know are an ancestor of at least one
     #   of the nodes in revs