Added GL_PROTOCOL as an ENV variable to gitCommand

With this MR we are now populating an ENV variable called GL_PROTOCOL when calling the gitCommand method. This will allow GitLab to allow or deny the request based on the allowed Git access protocols defined in the Application Settings.

This MR is needed for:

gitlab-shell!62 (merged)

gitlab-org/gitlab-ce!4696

gitlab-org/gitlab-ce#18601