Currently, git-webkit pr will unconditionally set the local main reference to point to the remote reference. This is so that we can seamlessly allow contributors to create a commit on `main` and then put that commit into a PR and let the engineer return their checkout to the canonical main. If we attempt to set the local main reference to the remote when it's already on the remote, though, that command will fail.
<rdar://problem/109860518>
Committed 264544@main (bb0606c): https://commits.webkit.org/264544@main