Make walker.fetch_ref() take a struct ref.

Make walker.fetch_ref() take a struct ref.

This simplifies a few things, makes a few things slightly more
complicated, but, more importantly, allows that, when struct ref can
represent a symref, http_fetch_ref() can return one.

Incidentally makes the string that http_fetch_ref() gets include "refs/"
(if appropriate), because that's how the name field of struct ref works.
As far as I can tell, the usage in walker:interpret_target() wouldn't have
worked previously, if it ever would have been used, which it wouldn't
(since the fetch process uses the hash instead of the name of the ref
there).

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