--- a/mercurial/localrepo.py	Sat Mar 02 09:41:17 2019 +0900
+++ b/mercurial/localrepo.py	Wed Feb 27 10:49:25 2019 +0100
@@ -752,7 +752,11 @@
                                      b'revlog.optimize-delta-parent-choice')
     options[b'deltabothparents'] = deltabothparents
 
-    options[b'lazydeltabase'] = not scmutil.gddeltaconfig(ui)
+    lazydeltabase = ui.configbool(b'storage',
+                                  b'revlog.reuse-external-delta-parent')
+    if lazydeltabase is None:
+        lazydeltabase = not scmutil.gddeltaconfig(ui)
+    options[b'lazydeltabase'] = lazydeltabase
 
     chainspan = ui.configbytes(b'experimental', b'maxdeltachainspan')
     if 0 <= chainspan: