This diff reverts 09c36781 and tweaks how some of our linting was
working.
I'm not actually sure what caused the regression that 09c36781 was
fixing - it was something to do with typescript being used to transpile
eslintrc.js, but that being excluded from the tsconfig for those
projects. I fixed that by removing `rootDir` from those, but that
revealed some other issues with files not getting ignored correctly.
I fixed the ignoring issue with a change I've wanted to make to these
scripts for a while: only running them on files that are actually
tracked by git, instead of on everything with a relevant extension. A
side effect of that is that we have to re-implement .eslintignore
support ourselves, but that's very straight forward: the `ignore`
package that eslint uses is very easy to include.
### Change Type
- [x] `internal` — Any other changes that don't affect the published
package (will not publish a new version)
### Test Plan
-
### Release Notes
[internal-only]
