Bump git version used from 2.14.3 to 2.16.2
In gitlab-com/migration#216 (closed), we found there are significant performance improvements in git 2.16 with regards to fetching a repo with many refs.
This MR bumps the version we test and the documentation which version to use.
Omnibus MR: omnibus-gitlab!2320 (merged)
Edited by Stan Hu