Update git and install git-lfs QA docker image

This MR updates git from 2.11.0 to 2.19.1 (latest in stretch-backports) and also install git-lfs 2.5.2 as it's needed in new Geo QA tests.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7516

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides