Bump git from 2.14.3 to 2.16.3

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.

The issue with stale worktrees we had bumping up to git 2.16 should be resolved via https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18095/diffs.

2.16.3 also has a significant memory leak for git fsck.

Closes #3375 (closed)