config: hide functions using `the_repository` by default

config: hide functions using `the_repository` by default

The config subsystem provides a bunch of legacy functions that read or
set configuration for `the_repository`. The use of those functions is
discouraged, and it is easy to miss the implicit dependency on
`the_repository` that calls to those functions may cause.

Move all config-related functions that use `the_repository` into a block
that gets only conditionally compiled depending on whether or not the
macro has been defined. This also removes all dependencies on that
variable in "config.c", allowing us to remove the definition of said
preprocessor macro.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>