diff -r 7a01778bc7b7 -r af2b5562fcaf mercurial/merge.py
--- a/mercurial/merge.py	Sun Sep 01 20:53:14 2019 +0200
+++ b/mercurial/merge.py	Mon Sep 23 21:29:53 2019 +0900
@@ -2250,23 +2250,17 @@
                    mergeancestor=mergeancestor, labels=labels)
 
 
-    potherp1 = False
     if keepconflictparent and stats.unresolvedcount:
         pother = ctx.node()
     else:
         pother = nullid
         parents = ctx.parents()
         if keepparent and len(parents) == 2 and pctx in parents:
-            if pctx == parents[1]:
-                potherp1 = True
             parents.remove(pctx)
             pother = parents[0].node()
 
     with repo.dirstate.parentchange():
-        if potherp1:
-            repo.setparents(pother, repo['.'].node())
-        else:
-            repo.setparents(repo['.'].node(), pother)
+        repo.setparents(repo['.'].node(), pother)
         repo.dirstate.write(repo.currenttransaction())
         # fix up dirstate for copies and renames
         copies.duplicatecopies(repo, repo[None], ctx.rev(), pctx.rev())