Commit 49416ad22 (completion: support excluding refs, 2016-08-24) made
possible to complete short refs with a '^' prefix.
Extend the support to full refs to make completing '^refs/...' work.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>