dirstate: warn if dirty when starting an edition

The dirstate should be clean before we start changing it. Otherwise we might
write unrelated changes. Having a dirty dirstate laying around is also
suspicious.

This is similar to what we do when opening a new transaction, but this time
this affect dirstate changes outside of a transaction.

mercurial/dirstate.py

@@ -190,6 +190,9 b' class dirstate:'
             raise error.ProgrammingError(msg)
 
         has_tr = repo.currenttransaction() is not None
+        if not has_tr and self._changing_level == 0 and self._dirty:
+            msg = "entering a changing context, but dirstate is already dirty"
+            raise error.ProgrammingError(msg)
 
         # different type of change are mutually exclusive
         if self._change_type is None: