completion: complete values of configuration variables after 'git -c var='

completion: complete values of configuration variables after 'git -c var='

'git config' expects a configuration variable's name and value in
separate options, so we complete values as they stand on their own on
the command line.  'git -c', however, expects them in a single option
joined by a '=' character, so we should be able to complete values
when they are following 'section.name=' in the same word.

Add new options to the __git_complete_config_variable_value() function
to allow callers to specify the current word to be completed and the
configuration variable whose value is to be completed, and use these
to complete possible values after 'git -c 'section.name=<TAB>'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>