This is particularly intended for invoking compiletest; the command line there
is long (3,350 characters on my system) and takes up a lot of screen real estate
for little benefit to the majority of those running bootstrap. This moves
printing it to verbose mode (-v must be passed) which means that it's still
possible to access when needed for debugging.
The main downside is that CI logs will by-default become less usable for
debugging (particularly) spurious failures, but it is pretty rare for us to
really need the information there -- it's usually fairly obvious what is being
run with a little investigation.
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