Add git clone with thinpack option to support self hosted azure devops

The program uses only the hostname to address the issue of requiring thin pack to perform a git clone operation on Azure DevOps, as shown in the code snippet below.

https://github.com/coder/envbuilder/blob/d045c1ce27a5995ac60409bd48ad8e90861b6bfd/git/git.go#L56-L78

However, since Azure DevOps supports self-hosted servers, the hostname may not necessarily be dev.azure.com. Therefore, it is desired to add a command-line option to enable this functionality.

I opened a pull request below.
#446

从 git/git.go 的第 56-78 行开始，这里由 hostname 决定是否使用 thin-pack 支持。检查 pull request #446 中提出的命令行选项，并验证它是否能为自托管的 Azure DevOps 服务器启用 thin-pack 克隆，而不依赖 dev.azure.com hostname。

由索引模型根据 Issue 内容生成。