reftable/basics: stop using `SWAP()` macro

reftable/basics: stop using `SWAP()` macro

Stop using `SWAP()` macro in favor of an open-coded variant of it. Note
that this also requires us to open-code the build assert that `SWAP()`
itself uses to verify that the size of both variables matches.

This is done to reduce our dependency on the Git codebase.

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