Emily Shaffer [off-list ref] writes:

Because all but 'post-commit' hooks are expected to make some state
change, force all but 'post-commit' hook to run in series.

OK.  And a sequence of hooks that run in series are expected to use
the output from the previous one as its input?

'post-commit'
"is meant primarily for notification, and cannot affect the outcome of
`git commit`," so it is fine to run in parallel.

Makes sense.