Method to force onchange commands to run

[merlinz01]

In the case of a git.UpToDate operation with an onchange that failed, the only way to rerun the failed onchange is to push another commit to the remote repository.

We should add a feature to mark a certain state operation as force-changed so that the onchange commands are re-run.

Perhaps in addition, track command failures and automatically force-change operations as needed.

[merlinz01]

Shouldn't be too hard once we have #69 implemented.

[merlinz01]

Should be a matter of specifying names of operations that should be considered to have changed.