rewrite revert command. fix issues 93, 123, 147. new version does these things: - saves backup copies of modified files (issue 147) - prints output like other commands, and errors when files not found (issue 123) - marks files added/removed (issue 93)

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