gdk install vs. gdk update

Whilst debugging #500 (comment 175059580), it was discovered that (using gitaly as the example project):

When a gdk install is executed, it checks out master
Where as a gdk update fetches and checks out the version defined in gitlab/GITALY_SERVER_VERSION

I feel installing and updating should both ensure gitlab/GITALY_SERVER_VERSION is checked out, thoughts?

Edited May 28, 2019 by Ash McKenzie