write_head_info(): handle "extra refs" locally

write_head_info(): handle "extra refs" locally

The old code basically did:

     generate array of SHA1s for alternate refs
     for each unique SHA1 in array:
         add_extra_ref(".have", sha1)
     for each ref (including real refs and extra refs):
         show_ref(refname, sha1)

But there is no need to stuff the alternate refs in extra_refs; we can
call show_ref() directly when iterating over the array, then handle
real refs separately.  So change the code to:

     generate array of SHA1s for alternate refs
     for each unique SHA1 in array:
         show_ref(".have", sha1)
     for each ref (this now only includes real refs):
         show_ref(refname, sha1)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>