It's a little surprising that git doesn't pass pre-commit hooks any information, like a list of which files were changed in the soon-to-be-made commit. git does so for pre-push, where it writes to a hook's stdin some information about the refs and remotes involved in the push.
I wonder if many pre-commit hooks, like the kind which run formatters, would be better off as `clean` filters, which run on files when they are staged. The filter mechanism makes it easier to apply just to the files which were changed. In the git docs, they even use a formatter (`indent`) as an example.
https://git-scm.com/book/ms/v2/Customizing-Git-Git-Attribute...