Since the failure is happening during install, I'm confident you're using git client plugin 2.7.2. Use 3.0.0-beta3 to see if it gives better results. It is built with Java 8 and JGit 4.11.0. The older version is JDK 7 compatible and is built with JGit 4.5.4
If the git client plugin fails to load, the git plugin which critically depends on it will also fail to load. If the git plugin fails to load, pandemonium ensues.