py3: handle keyword arguments correctly in simplemerge.py

Differential Revision: https://phab.mercurial-scm.org/D1641

mercurial/simplemerge.py

@@ -418,6 +418,8 b' def simplemerge(ui, localctx, basectx, o'
 
     The merged result is written into `localctx`.
     """
+    opts = pycompat.byteskwargs(opts)
+
     def readctx(ctx):
         # Merges were always run in the working copy before, which means
         # they used decoded data, if the user defined any repository