Reverse a git fetch

I'm not sure if there's an actual git command to do it, and this is a big hackish, but...

echo $OLD_COMMIT > .git/refs/remotes/origin/master

should work

This isn't a "reverse a git fetch" answer, but it seems your actual problem is how to programmatically "watch a repo for changes" without necessarily altering your repository in any way.

For this, you can use git fetch --dry-run. A dry run will not cause any changes to your repository, but if there are changes in the remote, then it will have some basic terminal output; if there are no changes there will not be any output. If you want to have this running as an automated script instead of a manual check, it should be relatively straightforward to create a simple bash script that tests git fetch --dry-run for output.

You want

git update-ref refs/remotes/origin/master refs/remotes/origin/master@{1}

update-ref wants the full spell on the ref it's updating because it's (much) lower level than the commands that respect ref-naming conventions.