Move some more code from the calling site into write_head_info(), and
inline add_alternate_refs() there. (Some more simplification is
coming, and it is easier if all this code is in the same place.)
Move some helper functions to avoid the need for forward declarations.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>