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

[CH-Chang]

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.

envbuilder/git/git.go

Lines 56 to 78 in d045c1c

	if parsed.Hostname() == "dev.azure.com" {
	// Azure DevOps requires capabilities multi_ack / multi_ack_detailed,
	// which are not fully implemented and by default are included in
	// transport.UnsupportedCapabilities.
	//
	// The initial clone operations require a full download of the repository,
	// and therefore those unsupported capabilities are not as crucial, so
	// by removing them from that list allows for the first clone to work
	// successfully.
	//
	// Additional fetches will yield issues, therefore work always from a clean
	// clone until those capabilities are fully supported.
	//
	// New commits and pushes against a remote worked without any issues.
	// See: https://github.com/go-git/go-git/issues/64
	//
	// This is knowingly not safe to call in parallel, but it seemed
	// like the least-janky place to add a super janky hack.
	transport.UnsupportedCapabilities = []capability.Capability{
	capability.ThinPack,
	}
	logf("Workaround for Azure DevOps: marking thin-pack as unsupported")
	}

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