Status bar button "Sign in on [Enterprise domain]" actually authorizes github.com

[callumlocke]

• Extension version: 0.13
• VSCode Version: 1.40.0
• OS: macOS Mojave

Steps to Reproduce:

1. Open a GitHub Enterprise project in VSCode.
2. Note the status bar button offers to "Sign in on [github enterprise domain]":
   
3. Click it, and it opens vscode-auth.github.com/authorize... in your browser, then when you click Continue, it wrongly takes you to to the authorization page on github.com. At the same time, it still misleadingly shows your Enterprise domain in the VSCode status bar:
   

[RMacfarlane]

@callumlocke Thanks for catching this! As a temporary work around, you can follow the directions here for manually adding an auth token: https://github.com/microsoft/vscode-pull-request-github/wiki#personal-access-token-authentication