New modified hook semantic

[Gustavo Niemeyer]

Hey James,

> Did you consider firing the old hook twice for joined? That would still

I didn't, but it feels like having the same file executing twice for
the same event would be a bit confusing.  It'd also preserve some of
the problems because it'd force people to do the "if [
$ENSEMBLE_CHANGED ...]" comparisons.

Given these issues, it seems like forcing people to manage their
dependencies externally (e.g. including the shell files, if necessary)
would be a better trade off.

-- 
Gustavo Niemeyer
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