cmake: silence "You are in 'detached HEAD' state" warning

cmake: silence "You are in 'detached HEAD' state" warning

git warns us when the repo is in 'detached HEAD' state, like:

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in
this state without impacting any branches by switching back to a branch.
...
Turn off this advice by setting config variable advice.detachedHead to
false

Signed-off-by: Kefu Chai <kchai@redhat.com>