completion: complete config variables names and values for 'git clone -c'

The previous commits taught the completion script how to complete configuration section, variable names, and their valus after 'git -c <TAB>', and with a bit of foresight encapsulated all that in a dedicated helper function. Use that function to complete the unstuck argument of 'git config -c|--config <TAB>', which expect configuration variables and values in the same 'section.name=value' form. Note that handling the struck argument for 'git clone --config=<TAB>' requires some extra care, so it will be done a separate patch. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>