path: hide functions using `the_repository` by default

path: hide functions using `the_repository` by default

The path subsystem provides a bunch of legacy functions that compute
paths relative to the "gitdir" and "commondir" directories of the global
`the_repository` variable. 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.

With `USE_THE_REPOSITORY_VARIABLE`, we have recently introduced a tool
that allows us to get rid of such functions over time. With this macro,
we can hide away functions that have such implicit dependency such that
other subsystems that want to be free of `the_repository` will not use
them by accident.

Move all path-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 "path.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>