Be kinder if the package is a requirements file

[peterbe]

So many times I've typed something like this:

▶ hashin --dry-run -u requirements/default.txt
Can not combine the --update-all option with a list of packages.

when I meant to type hashin --dry-run -u -r requirements/default.txt

It reminds me of how pip deals with pip install requirements.txt and thinks requirements.txt is a package name on PyPI.