Run phpstan on staged PHP source files

[pierlon]

Summary

This PR is a followup from #4441, which runs phpstan on staged PHP files within the includes or src folders.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[westonruter]

Excellent. Thank you.

[schlessera]

Did you test whether this works as intended? I can't imagine static analysis being useful is not run across the whole codebase. Contrary to PHPCS, the whole point of PHPStan is to look at the whole and reason about whether the flow across the codebase makes sense everywhere. If you just look at one file, you cannot really say much about typing consistency and similar issues.

[westonruter]

I did test. It caught an issue with a staged file.

[schlessera]

This goes counter to what the author of PHPStan recommends, though: https://medium.com/@ondrejmirtes/from-minutes-to-seconds-massive-performance-gains-in-phpstan-163be88d1519

[pierlon]

Ah I did not know that. So we can configure phpstan to run via the pre-commit hook then, instead of on staged files.