Keep tracked files instead of refreshing list of untracked files

[thatsmydoing]

https://github.com/paleite/eslint-plugin-diff/pull/27 introduced a massive performance regression with the recommended diff/diff setup. It effectively refreshes the list of untracked files for every unchanged file in the project.

Instead of constantly refreshing the tracked file list, it's better to get the list of tracked files once and we can simply check that a file is not in the list.

[TylerNRobertson]

Yeah noticing this as well, it makes the initial lint take really long to run, even when there is nothing to lint