renku undo command

[ciyer]

Is your feature request related to a problem? Please describe.
Undoing the effect of renku commands in the repository is tedious. It would nice if this were simpler.

Describe the solution you'd like
Add a command renku undo [--force]:

Undo the last renku operation

• if working dir is dirty, ask user to stash changes or clean
• if the last commit was not a renku operation, fail with a message that undo can only undo renku operations
• if the last commit has been pushed to its tracking branch (assuming this is easy to detect), fail unless the force flag is turned on.
• if all sanity checks pass, reset HEAD to HEAD~1 and clean working dir.

Describe alternatives you've considered
Do the same operation by hand.

[jirikuncar]

duplicate #338