config: make scope_name non-static and rename it

config: make scope_name non-static and rename it

To prepare for the upcoming --show-scope option, we require the ability
to convert a config_scope enum to a string.  As this was originally
implemented as a static function 'scope_name()' in
t/helper/test-config.c, we expose it via config.h and give it a less
ambiguous name 'config_scope_name()'

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