diff -r 4950ae4d034f -r 566daffc607d mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py	Wed Jul 19 13:17:49 2017 -0700
+++ b/mercurial/revlogutils/deltas.py	Wed Apr 03 11:21:27 2019 -0700
@@ -637,7 +637,7 @@
 
     deltas_limit = textlen * LIMIT_DELTA2TEXT
 
-    tested = set([nullrev])
+    tested = {nullrev}
     candidates = _refinedgroups(revlog, p1, p2, cachedelta)
     while True:
         temptative = candidates.send(good)