Upgrade git to 2.10
This MR will upgrade our bundled git in omnibus to 2.10.
See the analysis of possible issues and the explanation why it should not affect us: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1749#note_20104137 The idea after talking to @marin is that we are going to ship it to master and have plenty of time to test for possible regression.
cc @gitlab-build-team @domosapien
related to #1749 (closed)