remote: force completing --mirror= instead of --mirror

remote: force completing --mirror= instead of --mirror

"git remote --mirror" is a special case. Technically it is possible to
specify --mirror without any argument. But we will get a "dangerous,
deprecated!" warning in that case.

This new parse-opt flag allows --git-completion-helper to always
complete --mirror=, ignoring the dangerous use case.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>