replace-fork should not clear uncommitted changes (#22348)

The replace-fork script depends on ESLint to fix the reconciler imports — `.old` -> `.new` or vice versa. If ESLint crashes, it can leave the imports in an incorrect state. As a convenience, @bvaughn updated the script to automatically run `git checkout -- .` if the ESLint command fails. An unintended consequence of the strategy is that if the working directory is not clean, then any uncommitted changes will be lost. We need a better strategy for this that prevents the accidental loss of work. One option is to exit early if the working directory is not clean before you run the script, though that affects the usability of the script. An ideal solution would reset the working directory back to whatever state it was in before the script ran, perhaps by stashing all the changes and restoring them if the script aborts. Until we think of something better, I've commmented out the branch.