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As discussed in #13938, disabling tidy check on untracked file to reduce noise
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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#13938 (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because mocking `git` calls for the unit test is probably not worth the effort.
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Source-Repo: https://github.com/servo/servo
Source-Revision: afa55dcf5573d6b535a8e9681d42fe51e1ff01d1
7c0a3d7341