mergestate: add a constructor that reads state from disk

At the moment it's the same as just creating a new mergestate, but we'll soon
move the _read call out of __init__ and in here.

mercurial/merge.py

@@ -83,6 +83,12 b' class mergestate(object):'
         ms.reset(node, other)
         return ms
 
+    @staticmethod
+    def read(repo):
+        """Initialize the merge state, reading it from disk."""
+        ms = mergestate(repo)
+        return ms
+
     def __init__(self, repo):
         self._repo = repo
         self._dirty = False