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)

adding a
adding b
a
NONEXISTENT: No such file or directory
a
b
a: No such file or directory
NONEXISTENT: No such file or directory
b
a
NONEXISTENT: No such file in rev 14467d15ef43
a
b