The [manual page](man:gitnamespaces(7)) says:
> Git supports dividing the refs of a single repository into
> multiple namespaces, each of which has its own branches,
> tags, and `HEAD`.
Here, the mention of `HEAD` implies that requiring all
namespaced to start with
refs/namespaces/*/refs
is too strict, as there are legal namespaced refs such as
refs/namespaces/*/HEAD
which are not captured. Relax the corresponding condition, change
documentation and add a test to avoid regression.
