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

Make ls-remote ... 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>