context: move dirty() to committablectx

This is a pedantic move. It should be an error if dirty() is called on a
read-only context. Based on Mads Kiilerix's and my work at the sprint.

mercurial/context.py

@@ -323,9 +323,6 b' class basectx(object):'
     def hasdir(self, dir):
         return self._manifest.hasdir(dir)
 
-    def dirty(self, missing=False, merge=True, branch=True):
-        return False
-
     def status(self, other=None, match=None, listignored=False,
                listclean=False, listunknown=False, listsubrepos=False):
         """return status of files between two nodes or node and working
@@ -1569,6 +1566,9 b' class committablectx(basectx):'
         # from immediately doing so for subsequent changing files
         self._repo.dirstate.write(self._repo.currenttransaction())
 
+    def dirty(self, missing=False, merge=True, branch=True):
+        return False
+
 class workingctx(committablectx):
     """A workingctx object makes access to data related to
     the current working directory convenient.