Fixed ability to fetch private pull requests on Github

[StudioMaX]

With this change I added support of refs/pull on Github (https://help.github.com/articles/checking-out-pull-requests-locally/).
After this change this scheme will work:

• Organization "Company" has a private repository "Project"
• Mike has read-only access to this repository
• John has access to merge pull requests
• To make some changes in this project Mike creates a Fork of this repository. Then he commit his changes and creates a new Pull Request
• John could view this Pull Request on the original Project's repository (via the Web), but he cannot view the Fork of Mike, because this project is private
• But Github gives us ability to fetch changes from this PR as an external reference on the main repository (refs/pull/1/head)

[corpsee]

Thank you for request! I will review it tonight.