Fix dump-repo git init, fix wrong error type for NullDownloader (#20182)#20186

6543 merged 4 commits intogo-gitea:release/v1.17go-gitea/gitea:release/v1.17from

wxiaoguang:backport-dump-repo-git-initwxiaoguang/gitea:backport-dump-repo-git-initCopy head branch name to clipboard