merge: use the right ancestor when both sides copied the same file

The tests shows no real changes because of this ... but there must be some
weird corner cases where using the right ancestor for the merge planning is
better than using the wrong one.

mercurial/merge.py

@@ -388,9 +388,14 b' def manifestmerge(repo, wctx, p2, pa, br'
         if partial and not partial(f):
             continue
         if n1 and n2:
-            fla = ma.flags(f)
+            fa = f
+            a = ma.get(f, nullid)
+            if a == nullid:
+                fa = copy.get(f, f)
+                # Note: f as default is wrong - we can't really make a 3-way
+                # merge without an ancestor file.
+            fla = ma.flags(fa)
             nol = 'l' not in fl1 + fl2 + fla
-            a = ma.get(f, nullid)
             if n2 == a and fl2 == fla:
                 pass # remote unchanged - keep local
             elif n1 == a and fl1 == fla: # local unchanged - use remote