cmake: silence "detached HEAD" warning

cmake: silence "detached HEAD" warning

git complains when checking out a tag in "detached HEAD", like:

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them
...

but this does not help, as, in general, we don't hack fio in Ceph,
so disable this warning. and also clone the repo in shallow mode
for the same reason -- we don't care about the whole history of
fio repo. we just use it for testing.

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