Add hideRefs git configuration
Per https://gitlab.com/gitlab-org/gitlab-ce/issues/24329 and https://gitlab.com/gitlab-com/infrastructure/issues/765 .
Ask production team before 8.15 to give their opinion if it makes sense to make
"hideRefs=^refs/tmp/"
"hideRefs=^refs/keep-around/"
values default in the package. These values got enabled today in production so we can check on their effect before shipping to everyone.