completion: support full refs from remote repositories

completion: support full refs from remote repositories

When the __git_refs() completion helper function lists refs from a
local repository, it usually lists the refs' short name, except when
it needs to provide completion for words starting with refs, because
in that case it lists full ref names, see 608efb87 (bash: complete
full refs, 2008-11-28).

Add the same functionality to the code path dealing with remote
repositories, too.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>