merge: back out changeset a4ca0610c754 (parents order when grafting a merge) stable 5.1.2

merge: back out changeset a4ca0610c754 (parents order when grafting a merge)

Turns out it's not enough to just swap parents, because when we do, there are
unexpected bad side effects, such as a tracked file becoming untracked. These
side effects need more code to be handled properly, but it's not written yet.
Let's back this feature out from stable for now and some day implement it on
default instead.

--- a/mercurial/merge.py	Wed Sep 18 17:53:10 2019 +0700
+++ b/mercurial/merge.py	Fri Sep 20 23:31:03 2019 +0700
@@ -2249,23 +2249,17 @@
                    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())