fix: merge overrides with the same file patterns

[connorshea]

I created a very minimal reproduction test for this issue, and I believe I've fixed it, but it may have other consequences that need to be figured out and resolved. I'm pretty confident in this now

This was heavily AI-assisted after adding the tests, so I need to test it further, but based on the snapshots it seems like a pretty good solution.

See #240 for more info.

As an example of how much this fixes, I've been using the mastodon repo as a testing ground for the migrate package:

Results from running `yarn run oxlint` on the mastodon codebase after migrating it with `npx @oxlint/migrate eslint.config.mjs`

### v1.28.0 migrate:

Found 2540 warnings and 465 errors.

### main branch of migrate + overrides fix:

Found 67 warnings and 474 errors.

### From main migrate + overrides fix + main oxlint:

Found 67 warnings and 225 errors.

[connorshea]

this is kept because it is just one pass of the various steps, so we haven't removed files-only overrides yet.

[connorshea]

Here's an eslint config:

export default tseslint.config([
  react.configs.flat['jsx-runtime'],
  {
    plugins: {
      "react": react,
      '@typescript-eslint': tseslint.plugin,
    },
    languageOptions: {
      parser: tseslint.parser,
      parserOptions: {
        ecmaFeatures: {
          jsx: true,
        },
      },
    },
    rules: {
      'react/jsx-no-target-blank': [
        'error',
        {
          // Default value for this is false
          allowReferrer: true,
        },
      ],
    },
  },
  {
    files: ['**/*.ts', '**/*.tsx'],
    rules: {
      'react/jsx-no-target-blank': 'error'
    },
  },
]);

react/jsx-no-target-blank enforces that links with target='_blank' also have ``rel='noreferrer'` to avoid tracking and whatnot.

So, what happens when I have this code in app/javascript/mastodon/features/ui/components/link_footer.tsx, which does not have noreferrer?:

<a href='https://joinmastodon.org' target='_blank' rel='noopener'>
  <FormattedMessage id='footer.about' defaultMessage='About' />
</a>

The second config object inherits the base configuration's config for this rule, and so allows excluding noreferrer in tsx files lmao 😭

oxlint (or oxlint-migrate's interpretation of the eslint config I guess idk) does not do this. So that's one source of difference here.

[Sysix]

Thank you for working on this!

oxlint (or oxlint-migrate's interpretation of the eslint config I guess idk) does not do this. So that's one source of difference here.

Does it mean that the second block does not really override the settings, but only the severity (in ESLint)?
When ESLint does it but oxlint not, I would guess this is a bug in oxlint? And should later than be patched here too .)

[connorshea]

Does it mean that the second block does not really override the settings, but only the severity (in ESLint)? When ESLint does it but oxlint not, I would guess this is a bug in oxlint? And should later than be patched here too .)

Yeah, in ESLint it only overrides the severity and is probably, technically a bug in oxlint's config resolution behavior, unfortunately.

[connorshea]

After testing this on a decent number of configs, I think this is pretty good and ready to go. We still have the issue described above, but I think that's out-of-scope for this fix for now.

[connorshea]

@Sysix ready for review when you have time :)

[Sysix]

Thank you for the work (and the wait)! :)

[connorshea]

No worries, thank you!