Show git version on admin page

Before
![git-version-beore](/uploads/57444ad42220c9dfe8f187b8e399227f/git-version-beore.png)

After
![git-version-after](/uploads/cc1b52ae1a251c2c46d9626c040726ab/git-version-after.png)

gitlab-org/gitlab-ce#4077




See merge request !2101