Option for printing files changed by hook

[ArturGajowy]

Pre-commit detects changes made by hooks to the files in the repo. It'd be useful to have the option to print the changed files, as it's not always easy to make the tool we're running underneath do that for us.

My usage scenario would be having a scalafmt hook (a source formatter). Scalafmt doesn't output the files it modified, and has no option of requesting that.

I think though this is a generally useful feature, as many tools will be lacking in a similar way. Hence I propose print_changed_files: true to enable printing of the files changed by a hook.

[asottile]

there's a duplicate for this somewhere in the tracker -- maybe #681, we don't actually know the changed filenames without a complicated and slow lookup (we're currently using "does the git diff bytes change between invocations)

you can run git diff afterwards to kind of find the changes?

[asottile]

Or use --show-diff-on-failure in CI

[ArturGajowy]

Yeah, I thought pre-commit actually knows which files changed - but you've explained it doesn't.

I think what I'm looking for with this and #1086 is better suited to be done in a post-commit hook (mostly). So I ended up running the formatter in a post-commit hook, and making a fixup! commit with its changes. This:

• makes the change explicit
• allows ignoring it (till rebase / at all)
• doesn't require attempting the commit twice

[asottile]

cheers! thanks for the issue :)