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

Start in git/git.go at lines 56-78, where the hostname determines whether thin-pack support is used. Review the proposed command-line option in pull request #446 and verify that it enables thin-pack cloning for self-hosted Azure DevOps servers without relying on the dev.azure.com hostname.

Written by the indexing model from the issue text.