Reverse a git fetch

I have a thorough understanding of git, and the difference between pull, fetch, and merge. I have a remote that I track, fetch, and merge with occasionally, let say it's origin/master. What I'm looking to do is reverse the behavior of a 'git fetch'. It sounds goofy, but I want to un-update where my remote tracking branch points, to an older state, the state right before the last fetch. Is this possible? For example, lets say this is my workflow... ``` git show origin/master # shows commit abc123 git fetch # yay i got something! git show origin/master # shows commit def456 ``` mystery command goes here so that... ``` git show origin/master # shows commit abc123 ``` It's kind of a weird thing to want, but I have a crontab that watches a git repo to detect when there's something to fetch, and I'm having issues debugging the script that performs some actions based on this behavior. Instead of waiting for origin/master to change, I'd like to change it myself so I can debug my script!