Add support for garage.github.com

[mislav]

No description provided.

[samcoe]

Code looks good to me. Kind of odd that the check for garage comes before the call to normalizeHostname(). Not sure if that can/should be rectified.

Additionally, would like to see a test to two for this new behavior.

[mislav]

Kind of odd that the check for garage comes before the call to normalizeHostname().

Normalizing the hostname turns garage.github.com into github.com, so it's a bit too late to check for garage afterwards. However, normalizing garage.github.com as github.com elsewhere is beneficial for token reuse: since it's technically not possible to complete the web browser flow gh auth login -h garage.github.com, the regular github.com token will be used for authenticating to garage environment.