revset: make children() not look at p2 if null (issue5439)

Unlike p1 = null, p2 = null denotes the revision has only one parent, which
shouldn't be considered a child of the null revision. This was spotted while
fixing the issue4682 and rediscovered as issue5439.

mercurial/revset.py

@@ -723,12 +723,15 b' def _children(repo, subset, parentset):'
     cs = set()
     pr = repo.changelog.parentrevs
     minrev = parentset.min()
+    nullrev = node.nullrev
     for r in subset:
         if r <= minrev:
             continue
-        for p in pr(r):
-            if p in parentset:
-                cs.add(r)
+        p1, p2 = pr(r)
+        if p1 in parentset:
+            cs.add(r)
+        if p2 != nullrev and p2 in parentset:
+            cs.add(r)
     return baseset(cs)
 
 @predicate('children(set)', safe=True)

tests/test-revset.t

@@ -881,6 +881,13 b' test ancestors'
   $ log 'children(ancestor(4,5))'
   2
   3
+
+  $ log 'children(4)'
+  6
+  8
+  $ log 'children(null)'
+  0
+
   $ log 'closed()'
   $ log 'contains(a)'
   0