restic/.github/PULL_REQUEST_TEMPLATE.md

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What does this PR change? What problem does it solve?
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Checklist
---------

- [ ] I have read the [Contribution Guidelines](https://github.com/restic/restic/blob/master/CONTRIBUTING.md#providing-patches)
- [ ] I have enabled [maintainer edits for this PR](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork)
- [ ] I have added tests for all changes in this PR
- [ ] I have added documentation for the changes (in the manual)
- [ ] There's a new file in `changelog/unreleased/` that describes the changes for our users (template [here](https://github.com/restic/restic/blob/master/changelog/TEMPLATE))
- [ ] I have run `gofmt` on the code in all commits
- [ ] All commit messages are formatted in the same style as [the other commits in the repo](https://github.com/restic/restic/blob/master/CONTRIBUTING.md#git-commits)
- [ ] I'm done, this Pull Request is ready for review