merge: use public interface ms.localctx instead of ms._local

merge: use public interface ms.localctx instead of ms._local

--- a/mercurial/hg.py	Tue Jan 16 14:28:57 2018 +0100
+++ b/mercurial/hg.py	Thu Jan 18 19:40:17 2018 +0530
@@ -15,7 +15,6 @@
 
 from .i18n import _
 from .node import (
-    hex,
     nullid,
 )
 
@@ -858,7 +857,7 @@
         ms = mergemod.mergestate.read(repo)
         if ms.active():
             # there were conflicts
-            node = hex(ms._local)
+            node = ms.localctx.hex()
         else:
             # there were no conficts, mergestate was not stored
             node = repo['.'].hex()