When we checkout a PR, we currently fetch the entirity of a contributor's fork. We shouldn't do that, we only need to fetch the PR branch we're testing.
<rdar://problem/108477382>
Pull request: https://github.com/WebKit/WebKit/pull/13125
Committed 263347@main (f5ddb76a9b06): <https://commits.webkit.org/263347@main> Reviewed commits have been landed. Closing PR #13125 and removing active labels.