Implementation of Revert/Rollback

[ty1824]

I've added a comment on the already-open issue: #583 but I figured that if discussions are new, I might as well add one here!

I'm not a long-term user, I adopted go-git recently as part of a project I'm working on. I'm trying to build a tool to help coordinate commands across multiple repositories. I have implemented a few operations, but I am trying to implement a cross-repo merge command.

One challenge is that if any of the individual merges fail, all of them must be rolled back (the operation must be atomic).

I'm not quite sure what the ideal approach to this would be.

• Attempt to emulate the behavior of git revert
• Attempt to rollback (reset the target branch to the commit prior to the merge)
• Something else?

This tool should be considered to have "administrator-level" access to the origin, so while there may be concerns about the approach, that's separate from the current issue :)