graft: never set both parents equal in the dirstate (issue6098)

merge.graft() can set both parents equal in the dirstate when
keepparent=True. We don't seem to set that in core, but the evolve
extension does use it. So I couldn't figure out a way to add a test
for this patch in core.

Differential Revision: https://phab.mercurial-scm.org/D7549

mercurial/merge.py

@@ -2626,6 +2626,9 b' def graft('
         if keepparent and len(parents) == 2 and base in parents:
             parents.remove(base)
             pother = parents[0].node()
+    # Never set both parents equal to each other
+    if pother == pctx.node():
+        pother = nullid
 
     with repo.dirstate.parentchange():
         repo.setparents(pctx.node(), pother)