contrib/completion: extract common diff/difftool options

contrib/completion: extract common diff/difftool options

difftool parses its own options and then passes the remaining options
onto diff. As a result, they share common command-line options. Instead
of duplicating the list, use a shared $__git_diff_difftool_options list.

The completion for diff is missing --relative and the completion for
difftool is missing --no-index. Add both of these to the common list.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>