_git_config() contains two enormous case statements, one to complete
configuration sections and variable names, and the other to complete
their values.
Split these out into two separate helper functions, so in the next
patches we can use them to implement completion for 'git -c <TAB>'.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>