Handle branch name with prefix in GitHub migration (#20357)
Handle branch name with prefix in GitHub migration (#20357)

GitHub allows releases with target commitish `refs/heads/BRANCH`, which
then causes issues in Gitea after migration. This fix handles cases that
a branch already has a prefix.

Fixes #20317

Co-authored-by: wxiaoguang <afddda5b6ea4f5ea4386c1c8946f240cce9ffa2e@gmail.com>
go-gitea/gitea,go-gitea/gitea,go-gitea/gitea