Hide files excluded by .gitignore

[ti-mo]

Fixes #14

This commit adds support for hiding files from output that have been excluded
by .gitignore and friends. I've implemented a few version of this and settled
on just `git ls-files` since it's likely going to be the most correct and
maintainable solution.

I've tried github.com/boyter/gocodewalker but it's a complex piece of machinery
and was much slower on a repo of 15k files (Cilium) than just git ls-files.
I also tried out github.com/ianlewis/go-gitignore directly, but it doesn't pick
up .gitignore files in subdirs automatically, nor the system-wide gitignore.

I figured since the overwhelming majority of users will be running this in CI
where Git will always be present, relying on the canonical .gitignore
implementation (git itself) is the safest option.