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