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.

[rwese]

Imho relying on the git cli command is not optimal.

Refactoring findRepositoryRoot would completely remove the need for exec breakout.

[ti-mo]

Honestly, I wouldn't reach for something much more complex. It's pretty involved to implement correctly, although the codeowners lib already has fnmatch -> regex compilation, which is a large chunk of what's needed. Hard to tell if it works exactly like git's, though.

If we use git as the source of truth for files to manage, there shouldn't be any surprises, it should just work. If someone wants to swap in a pure-Go implementation later, that's fine, but I personally don't think it's worth the engineering effort for a small tool like this.

[rwese]

I am not the owner of this repo, but my 2 cents after reviewing the pr:

1. test would be nice, could be accomplished by adding testdata/.gitignore and force add a testdata/ignored-file then have an example to check this.
2. having a flag to enable it, to not break backward compatibility.