release.sh: Don't install git if already present

Prior to this commit, we install git from ports if there is a ports
tree available and git is not installed, and we install git from pkg
otherwise -- including the case where git is already installed.

Rework the logic to not (re)install git at all if it is already
installed.

MFC after: 3 days

(cherry picked from commit 6baddb6b117664f51e2b1d2427fced946913a08f)