错误：远程 HEAD 引用不存在的 ref，无法检出
如果已在 您的 GitHub Enterprise Server 实例 上删除仓库的默认分支，会发生此错误。
检测此错误很简单；当您尝试克隆以下仓库时，Git 会警告您：
$ git clone https://主机名/user/repo.git
# Clone a repo
> Cloning into 'repo'...
> remote: Counting objects: 66179, done.
> remote: Compressing objects: 100% (15587/15587), done.
> remote: Total 66179 (delta 46985), reused 65596 (delta 46402)
> Receiving objects: 100% (66179/66179), 51.66 MiB | 667 KiB/s, done.
> Resolving deltas: 100% (46985/46985), done.
> warning: remote HEAD refers to nonexistent ref, unable to checkout.要修复此错误，您需要成为 您的 GitHub Enterprise Server 实例 上仓库的管理员。 您需要更改仓库的默认分支。
之后，您可以从命令行获取所有可用分支的列表：
$ git branch -a
# Lists ALL the branches
> remotes/origin/awesome
> remotes/origin/more-work
> remotes/origin/new-master然后，您可以切换到新分支：
$ git checkout new-master
# Create and checkout a tracking branch
> Branch new-master set up to track remote branch new-master from origin.
> Switched to a new branch 'new-master'