Node 27 and later publish Alpha prereleases, like `v27.0.0-alpha.1.0.0`, in the main `index.tab`,
so `nvm install node` or `nvm install 27` would otherwise pick one.
Every version is in exactly one category: `alpha` and `lts` or `non-lts` (by index.tab's LTS column) on the node mirror, and `iojs` on the io.js mirror.
Commands select from every category but `alpha`, unless `$NVM_ONLY` lists others,
and every command that selects or lists versions takes `--only=<categories>` to override both;
an exact alpha needs `alpha` selected, like any other.
An LTS selection, like `--lts` or an `lts/` version, can't combine with an `--only=` that excludes `lts`.
Telling installed `lts` and `non-lts` versions apart uses the recorded LTS status, and fails rather than guess without it.
`nvm alias` also stores `--only=`, so `nvm alias default --only=alpha node` makes the latest installed alpha the default, including in a new shell.
Jordan Harband
3fda4d5542