Log git-{upload-pack,receive-pack} requests

This builds on the work in !366 (merged).

Example log for git-upload-pack

{
  "command": "git-upload-pack",
  "git_protocol": "",
  "gl_project_path": "root/simple-ci",
  "gl_repository": "project-230",
  "level": "info",
  "msg": "executing git command",
  "time": "2020-03-08T08:22:19Z",
  "user_id": "user-1",
  "username": "root"
}