completion: fast initial completion for config 'remote.*.fetch' value

Refspecs for branches in a remote repository start with 'refs/heads/', so completing those refspecs with 'git config remote.origin.fetch <TAB>' always offers 'refs/heads/' first, because that's the unique part of the possible refspecs. But it does so only after querying the remote with 'git ls-remote', which can take a while when the request goes through some slower network to a remote server. Don't waste the user's time and offer 'refs/heads/' right away for 'git config remote.origin.fetch <TAB>'. The reason for putting 'refs/heads/' directly into COMPREPLY instead of using __gitcomp() is to avoid __gitcomp() adding a trailing space. Signed-off-by: SZEDER Gábor <szeder@ira.uka.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>