How can I uncommit the last commit in a git bare repository?

You can use the git update-ref command. To remove the last commit, you would use:

$ git update-ref HEAD HEAD^

Or if you’re not in the branch from which you cant to remove the last commit:

$ git update-ref refs/heads/branch-name branch-name^

You could also pass a sha1 if you want:

$ git update-ref refs/heads/branch-name a12d48e2

See the documentation of the git-update-ref command.