replace-fork: Cleanup after failure if no unstaged changes

[bvaughn]

The replace-fork script shouldn't override pending changes if ESLint failed, but it's also not great for it to leave the workspace in a broken state. This PR introduces a compromise approach:

1. Prompt and warn if there are unstaged changes when running the script.
2. If the ESLint fails and there are no unstaged changes, cleanup the bad files. Otherwise warn that the cleanup needs to be done manually.

Follow up to #22348

[sizebot]

Comparing: 293059e...267c68f

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	127.38 kB	127.38 kB	=	40.58 kB	40.58 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	130.20 kB	130.20 kB	=	41.50 kB	41.50 kB
facebook-www/ReactDOM-prod.classic.js	=	405.21 kB	405.21 kB	=	74.99 kB	74.99 kB
facebook-www/ReactDOM-prod.modern.js	=	393.80 kB	393.80 kB	=	73.28 kB	73.28 kB
facebook-www/ReactDOMForked-prod.classic.js	=	405.21 kB	405.21 kB	=	74.99 kB	74.99 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 267c68f