archive: remove the_repository global variable

archive: remove the_repository global variable

As part of the effort to get rid of global state due to the global
the_repository variable, replace the_repository with the repository
argument that gets passed down through the builtin function.

The repo might be NULL, but we should be safe in write_archive() because
it detects if we are outside of a repository and calls
setup_git_directory() which will error.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>