lib/ordered_tree: factorize `write_line` so subclasses can redefine it.
lib/ordered_tree: factorize `write_line` so subclasses can redefine it.

Signed-off-by: Jean Privat <51f8b1fa9b424745378826727452997ee2a7c3d7@pryen.org>

privat/nit,privat/nit,jpcaissy/nit,privat/nit,jpcaissy/nit,privat/nit,jcbrinfo/nit,jcbrinfo/nit,privat/nit,jpcaissy/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,jcbrinfo/nit,jpcaissy/nit,jpcaissy/nit,jpcaissy/nit,jpcaissy/nit,privat/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,jpcaissy/nit,jcbrinfo/nit,jcbrinfo/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,privat/nit