Accept directories as input

[baggiponte]

Description

When I run blacken docs src I get an error because it is a directory. I would say it'd be a nice addition if this could work by default. I checked the source code and this could easily be achieved doing something like this:

from pathlib import Path


def main(argv: Sequence[str] | None = None) -> None:
    """Main entry point for the program."""
    parser = argparse.ArgumentParser()
    parser.add_argument("filenames", nargs="*")
    args = parser.parse_args(argv)

    for filename in args.filenames:
        retv = 0
        for file in Path(filename).rglob("*"):
            ...

If you agree, I would like to propose a PR.

[adamchainz]

I'd rather avoid adding and maintaining undifferentiated features like recursion (the next request would be glob patterns, ...). It's not hard to compose shell globbing or git ls-files + xargs to apply to all the files you want. I've added some docs in #269 that cover this.