[feature request] Support adding a review comment on a line not part of the diff

[planetmarshall]

Feature Request

The Create a review comment for a pull request and comments parameter of Create a review for a pull request does not allow creating a review comment on a line of a changed file that is not part of the diff. Any attempt to do so results in the error -

Pull request review thread line must be part of the diff

However, I am able to do this manually as part of a review via the WebUI.

A use case is automatically adding comments from a linter to the review. In order to do this a user of the API has to calculate which lines from the linter results are part of the diff, which is non-obvious.

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