Bump GitLab CI image to use git 2.16
git 2.15.0 has some significant performance improvements when running git fetch on a repository that has lots of references. See
gitlab-com/migration#216 (comment 60672987) for more details.
- Omnibus MR: omnibus-gitlab!2320 (merged)
- Gitaly MR: gitaly!607 (merged)
- Build images MR: gitlab-build-images!79 (merged)