refs: move `refs_head_ref_namespaced()`

refs: move `refs_head_ref_namespaced()`

The function `refs_head_ref_namespaced()` is somewhat special when
compared to most of the other functions that take a callback function:
while `refs_for_each_*()` functions yield multiple refs,
`refs_heasd_ref_namespaced()` will only yield at most the HEAD ref of
the current namespace. As such, the function is related to
`refs_head_ref()` and not to the for-each functions.

Move the function to be located next to `refs_head_ref()` to clarify.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>