refs: move head_ref{,_submodule} to the common code

refs: move head_ref{,_submodule} to the common code

These don't use any backend-specific functions.  These were previously
defined in terms of the do_head_ref helper function, but since they
are otherwise identical, we don't need that function.

Signed-off-by: David Turner <redacted>
Reviewed-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>