When comparing branches, only offer those branches to use as a base
where the repository allows pull requests. Those that do not allow pull
request would result in a 404, so offering them as an option would be
misleading.
Refs: https://codeberg.org/forgejo/forgejo/pulls/2194
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
(cherry picked from commit 022d0e0d71a92c31302176c5c8ba1e7169bbbf3e)
(cherry picked from commit 957990b36a25d0e51d9b75432a577dd63fb6dad2)
(cherry picked from commit 6d2df728257922cc716fed8a172ed69adc8d46d3)
