Revert 09c36781 & tweak linting (#1501)

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]

scripts/lint.ts

@@ -1,33 +1,68 @@
-import { join, relative } from 'path'
-
+import ignore from 'ignore'
+import * as path from 'path'
 import { exec } from './lib/exec'
-import { REPO_ROOT } from './lib/file'
+import { REPO_ROOT, readFileIfExists } from './lib/file'
+
+const ESLINT_EXTENSIONS = ['js', 'jsx', 'ts', 'tsx']
+const PRETTIER_EXTENSIONS = ['js', 'jsx', 'ts', 'tsx', 'json']
 
 async function main() {
 	const shouldFix = process.argv.includes('--fix')
 
+	const lsFiles = await exec('git', ['ls-files', '.'], {
+		processStdoutLine: () => {
+			// don't print anything
+		},
+	})
+
+	const filesByExtension = new Map<string, string[]>()
+	for (const file of lsFiles.trim().split('\n')) {
+		const ext = file.split('.').pop()
+		if (!ext) continue
+		let files = filesByExtension.get(ext)
+		if (!files) {
+			files = []
+			filesByExtension.set(ext.toLowerCase(), files)
+		}
+		files.push(file)
+	}
+
+	let prettierFiles = PRETTIER_EXTENSIONS.flatMap((ext) => filesByExtension.get(ext) ?? [])
+	let eslintFiles = ESLINT_EXTENSIONS.flatMap((ext) => filesByExtension.get(ext) ?? [])
+
+	const relativeCwd = path.relative(REPO_ROOT, process.cwd())
+
+	const prettierIgnoreFile = await readFileIfExists(path.join(REPO_ROOT, '.prettierignore'))
+	if (prettierIgnoreFile) {
+		prettierFiles = prettierFiles
+			.map((f) => path.join(relativeCwd, f))
+			.filter(ignore().add(prettierIgnoreFile).createFilter())
+			.map((f) => path.relative(relativeCwd, f))
+	}
+
+	const eslintIgnoreFile = await readFileIfExists(path.join(REPO_ROOT, '.eslintignore'))
+	if (eslintIgnoreFile) {
+		eslintFiles = eslintFiles
+			.map((f) => path.join(relativeCwd, f))
+			.filter(ignore().add(eslintIgnoreFile).createFilter())
+			.map((f) => path.relative(relativeCwd, f))
+	}
+
 	try {
-		await exec(
-			'yarn',
-			[
-				'run',
-				'-T',
-				'prettier',
-				shouldFix ? '--write' : '--check',
-				// we have to run prettier from root so it picks up the ignore file
-				join(relative(REPO_ROOT, process.cwd()), '**', '*.{ts,tsx,js,jsx,json}'),
-			],
-			{ pwd: REPO_ROOT }
-		)
+		await exec('yarn', [
+			'run',
+			'-T',
+			'prettier',
+			shouldFix ? '--write' : '--check',
+			...prettierFiles,
+		])
 		await exec('yarn', [
 			'run',
 			'-T',
 			'eslint',
 			'--report-unused-disable-directives',
-			'--ignore-path',
-			join(REPO_ROOT, '.eslintignore'),
 			shouldFix ? '--fix' : null,
-			'.',
+			...eslintFiles,
 		])
 	} catch (error) {
 		process.exit(1)