Your problem is not undoing merges, but allowing people to merge that shouldn't. A good solution like GitHub Enterprise or Stash will allow you to use pull req

Your problem is not undoing merges, but allowing people to merge that shouldn't. A good solution like GitHub Enterprise or Stash will allow you to use pull requests, so you can require that code gets reviewed by a third-party before merging. In the case of Atlassian Stash, you can set branch permissions, which allow only certain users to commit/merge to that branch, and you can also require that a merge request be reviewed by specific individuals before the system allows it to be merged. Preventing the merge problem in the first place is a much better approach than trying to fix it afterwards.