https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24034 introduced
a regression where only the first 20 branches were used to determine
whether a branch has been merged because the pagination was applied
incorrectly. Requesting the second page of branches via the API would
always have the wrong merge status. We fix this by properly paginating
the branches before requesting their merge status.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56250