`rtx upgrade` ref support

[jdx]

rtx upgrade was added in #534 but it's intended to be used for version number upgrades (e.g.: 1.0.0 -> 1.0.1). It doesn't know how to upgrade ref tags that don't change, e.g.: rtx upgrade neovim@ref:master. This is because the ref doesn't change, it's still "master" to rtx.

For now you will need to run this to upgrade these:

$ rtx uninstall neovim && rtx i neovim@ref:master

I haven't thought too deeply about what would need to happen here but I suspect it will require some modification to the plugin, maybe a bin/upgrade script would need to be added. Or perhaps rtx could dereference "master" into a sha or something.