Test if plugin can be updated.

[fazibear]

Just in case of

error: cannot pull with rebase: You have unstaged changes.
error: please commit or stash them.

[Stratus3D]

Thanks for the PR @fazibear but this is something we've discussed on #124. I think it will be better to address the root cause (permission changes) rather than printing an error. Ideally a plugin update should only fail due to network issues. I think currently the error makes it to STDOUT when that happens.

[fazibear]

I saw it, but testing it like this #138 is pointless because it is already changed during plugin-add process https://github.com/asdf-vm/asdf/blob/master/lib/commands/plugin-add.sh#L19

[Stratus3D]

@fazibear correct, and that has been fixed with #154.