Fix pr checkout OWNER:BRANCH when maintainer can modify

We did not use to request the necessary GraphQL fields to determine
whether the PR in question can be modified by maintainers (i.e. pushed
back to).

api/queries_pr.go

@@ -540,8 +540,12 @@ func PullRequestForBranch(client *Client, repo ghrepo.Interface, baseBranch, hea
 					headRepositoryOwner {
 						login
 					}
+					headRepository {
+						name
+					}
 					isCrossRepository
 					isDraft
+					maintainerCanModify
 					reviewRequests(first: 100) {
 						nodes {
 							requestedReviewer {