Make setting up git-hooks-core idempotent

[Kehrlann]

Running the setup multiple times was failing on High Sierra. First run was fine, subsequent runs are failing due to $HOME/workspace/git-hooks-core being present already. Also, we were littering the git config with multiple core.hookspath values pointing to that directory.

Changed the script scripts/common/git.shso that it :

• only clones the pivotal-cf/git-hooks-core repo on the first run
• only adds the git config core.hookspath on the first run
• on subsequent runs, pulls the latest changes

Note: one assumption we made is that you will not be changing the git-hooks-core repo locally ; this is why we allow a git pull -r on it.

[nertzy]

I will merge this once I regain my committer access.

[onlyawave]

Merged