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)

merge: warning: conflicts during merge
merging a
merging a failed!
e7fe8eb3e180+0d24b7662d3e+ tip
something else
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something
M a