Use .gitignore for excluding files, if present

[DylanSp]

It'd be handy to use .gitignore to exclude files from checks, instead of having to duplicate those exclusions in the exclude setting.

[tzachov]

I actually don't like the idea of using the same file for multiple purposes. Let me think about it.

[DylanSp]

It's not too uncommon for tools that manage source code to respect .gitignore; off the top of my head, ripgrep does it out of the box, ESLint can use it with --ignore-path .gitignore.