Make ls-remote http://... list HEAD, like for git://...

This makes a struct ref able to represent a symref, and makes http.c
able to recognize one, and makes transport.c look for "HEAD" as a ref
in the list, and makes it dereference symrefs for the resulting ref,
if any.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.h

@@ -62,6 +62,8 @@ int check_ref_type(const struct ref *ref, int flags);
  */
 void free_refs(struct ref *ref);
 
+int resolve_remote_symref(struct ref *ref, struct ref *list);
+
 /*
  * Removes and frees any duplicate refs in the map.
  */