completion: difftool takes both revs and files

'git difftool' is clearly a frontend to 'git diff' and is used in exactly the same way, but it uses a misleadingly named completion function __git_complete_file. It happens to work only because it calls __git_complete_revlist_file that completes both revs and paths. Change it to use __git_complete_revlist_file, just like 'git diff'. Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>