git config: don't allow --get-color* and variable type

git config: don't allow --get-color* and variable type

Doing so would be incoherent since --get-color would pick a color slot
and ignore the variable type option (e.g. --bool), and the type would
require a variable name.

Suggested by Junio C Hamano.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>