mergestate: move _read() call to read constructor

With this patch, mergestate.clean() will no longer abort when it encounters an
unsupported merge type. However we hold off on testing it until backwards
compatibility is in place.

mercurial/merge.py

@@ -87,12 +87,15 b' class mergestate(object):'
     def read(repo):
         """Initialize the merge state, reading it from disk."""
         ms = mergestate(repo)
+        ms._read()
         return ms
 
     def __init__(self, repo):
+        """Initialize the merge state.
+
+        Do not use this directly! Instead call read() or clean()."""
         self._repo = repo
         self._dirty = False
-        self._read()
 
     def reset(self, node=None, other=None):
         self._state = {}