`default` was made legacy back in 2011 with
5be66da9f4 (the command was renamed from
`rbenv-default` to `rbenv-global`, and so the global file was renamed
from `$RBENV_ROOT/default` to `$RBENV_ROOT/global` (the latter taking
precedence)
`global` was then made legacy about a month later in Sep 2011 when the
preferred filename was changed to `$RBENV_ROOT/version`.
Can be used for `.ruby-version` file lookup in the ancestry of a
specific directory. In this mode of operation, global version files
aren't taken into consideration, and the command fails unless a local
version file was found.
