Bump GitLab CI test image to use git v2.16
Bump GitLab CI test image to use git v2.16

git 2.15.0 has some significant performance improvements when running `git
fetch` on a repository that has lots of references. See
https://gitlab.com/gitlab-com/migration/issues/216#note_60672987 for more
details.

dreampet/gitlab,stoplightio/gitlabhq,iiet/iiet-git,iiet/iiet-git,dreampet/gitlab,dreampet/gitlab,mmkassem/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq