Added logic to automatically detect smart Git hosts that allow shallow
cloning. This is done by sending an `ls-remote` request to the server
and then evaluating the returned HTTP header fields. For this, Curl
verbose logging is enabled for the `ls-remote` request, since Curl
verbose logging sends the returned HTTP headers to `stderr`.
If the `stderr` output contains the desired header
Content-Type: application/x-git-upload-pack-advertisement
then the server supports shallow cloning.
This approach uses Git and Curl for the heavy lifting. Instead of
implementing the request to the server using a simple HTTP client, Git
is used, since it takes care of authentication using stored credentials.
The used approach should also work for BitBucket, which only sends the
Content-Type header when a specific user agent is used. Using Git to
make the request enables this behavior.
The function to detect the smart Git host
(`GitRemoteResolver.prototype._supportsShallowCloning`) returns a
promise that is resolved when the server's request is evaluated. The
promise handling required an addition to `GitHubResolver.js` - to always
resolve the promise to `true`, since GitHub supports shallow cloning.
Added test cases to verify the new functionality.
