revert a commit

[extrawurst]

the revlog should feature a command to revert the marked commit. if this works without conflicts commit it with a default commit msg if not switch to the status tab where the uncommitted changes are visible then.

Reference impl in c#

git2-rs support: https://docs.rs/git2/0.13.23/git2/struct.Repository.html#method.revert

[extrawurst]

Supersedes #140