diff -r f8db458651c8 -r f39efa885a6d mercurial/revlog.py
--- a/mercurial/revlog.py	Fri Jul 20 13:32:17 2018 +0200
+++ b/mercurial/revlog.py	Fri Jul 20 13:34:48 2018 +0200
@@ -2103,7 +2103,10 @@
         deltap = self.deltaparent(rev)
         if deltap == nullrev:
             return True
-        return False
+        p1, p2 = self.parentrevs(rev)
+        if deltap in (p1, p2):
+            return False
+        return self.issnapshot(deltap)
 
     def revdiff(self, rev1, rev2):
         """return or calculate a delta between two revisions