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

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>