Fix file-changed-to-dir and dir-to-file commits (issue660). Allow adding to dirstate files that clash with previously existing but marked for removal. Protect from reintroducing clashes by revert. This change doesn't address related issues with update. Current workaround is to do "clean" update by manually removing conflicting files/dirs from working directory.

%% Should show unknown
? unknown
removing b
%% Should show unknown and b removed
R b
? unknown
%% Should show a and unknown
a
unknown