added unittest for _download_git

[kanikas3]

Summary of changes

• Added unittest for _download_git function

Closes
#1504

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[pganssle]

@kanikas3 and I spoke about the changes that needed to be made at the NYC PyPA sprint, but I've added explicit review comments here just as a reminder and a documentation of what needs to be done.

Overall, the goal of this test should be to make calls into the public interface and verify that they make the correct system calls. To some extent the fact that we're mocking out os.system is testing an implementation detail (since we could easily use subprocess instead), but I think it's a necessary evil if this isn't going to be an implementation test.

[pganssle]

@kanikas3 @vitoace Ping - are you still coming back to this?

[vitoace]

Sorry, probably not.

Thanks Paul for the review. Given the waned interest, I've gone ahead and addressed your concerns.

[jaraco]

Thanks @kanikas3 and @vitoace for your contributions here. Although I ended up rewriting some of the changes, your initial contribution saved me a great deal of time placing the test and understanding the basic usage we wished to test. Your commit will be included in the history and I encourage you to look at the subsequent changes for your own edification. Thanks again.