Some hooks take an appreciable amount of time to complete, so if users could see output as it is produced it would give them an opportunity to take early action. A good example is mypy, which must run on the entire codebase to be truly useful and so often produces a trickle of errors or warnings over a long period of time. If pre-commit is being run in verbose mode, I don't see any downside to showing immediate updates of the hook process's stdout and stderr, since it will be displayed eventually anyway.

This is similar to #872 but I'm not interested in interacting with stdin, and I'm suggesting that we do this by default if verbose is enabled, rather than introducing new config options. This works nicely with #695.