completion: support 'git config --local'

completion: support 'git config --local'

This needs to be done in two places: __git_config_get_set_variables to
allow clever completion of "git config --local --get foo<tab>", and
_git_config to allow "git config --loc<tab>" to complete to --local.

While we're there, change the order of options in the code to match
git-config.txt.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>