context: implement mergestate() method

This will let us have the mergestate storage be controlled by the context. In
particular, for working contexts we should use the existing mergestate, but
for overlay contexts it's inappropriate to drop files in .hg/merge.

Differential Revision: https://phab.mercurial-scm.org/D8551

mercurial/context.py

@@ -34,6 +34,7 b' from . import ('
     error,
     fileset,
     match as matchmod,
+    mergestate as mergestatemod,
     obsolete as obsmod,
     patch,
     pathutil,
@@ -474,6 +475,12 b' class basectx(object):'
 
         return r
 
+    def mergestate(self, clean=False):
+        """Get a mergestate object for this context."""
+        raise NotImplementedError(
+            '%s does not implement mergestate()' % self.__class__
+        )
+
 
 class changectx(basectx):
     """A changecontext object makes access to data related to a particular
@@ -2003,6 +2010,11 b' class workingctx(committablectx):'
 
         sparse.aftercommit(self._repo, node)
 
+    def mergestate(self, clean=False):
+        if clean:
+            return mergestatemod.mergestate.clean(self._repo)
+        return mergestatemod.mergestate.read(self._repo)
+
 
 class committablefilectx(basefilectx):
     """A committablefilectx provides common functionality for a file context