Can I rebase a Git branch without modifying my working copy?

Since git 2.5, an even better solution is to use a second worktree.

A git repository can support multiple working trees, allowing you to check out more than one branch at a time.

$ git worktree add ../second-copy experimental
$ cd ../second-copy/
$ git rebase master experimental

And that’s it. Afterwards, you can rm -rf second-copy if you want, or keep it for more rebases in the future.

$ git rebase master experimental