config: add '--show-scope' to print the scope of a config value

config: add '--show-scope' to print the scope of a config value

When a user queries config values with --show-origin, often it's
difficult to determine what the actual "scope" (local, global, etc.) of
a given value is based on just the origin file.

Teach 'git config' the '--show-scope' option to print the scope of all
displayed config values.  Note that we should never see anything of
"submodule" scope as that is only ever used by submodule-config.c when
parsing the '.gitmodules' file.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>