merge: respect parents order when using `graft` on a merge stable

merge: respect parents order when using `graft` on a merge

The previous code did not record the index of the replaced parent. It was always
using the "graft" destination as `p1`. This could switch parents order in some
situation (eg: some of the evolve evolving merge case). Recording and using the
information fixes the issue in evolve.

We are not aware of core commands calling graft in that fashion, so we could not
build a simple test case for it using core commands.

--- a/mercurial/merge.py	Sat Sep 07 14:51:18 2019 +0200
+++ b/mercurial/merge.py	Mon Sep 09 17:32:21 2019 +0200
@@ -2249,17 +2249,23 @@
                    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[0]:
+                potherp1 = True
             parents.remove(pctx)
             pother = parents[0].node()
 
     with repo.dirstate.parentchange():
-        repo.setparents(repo['.'].node(), pother)
+        if potherp1:
+            repo.setparents(pother, repo['.'].node())
+        else:
+            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())