Ignore all files by default.

https://wiki.archlinux.org/index.php/AUR_submission_guidelines

> Tip: To keep the working directory and commits as clean as possible,
> create a gitignore(5) that excludes all files and force-add files as
> needed.

.gitignore

@@ -1,5 +1 @@
-*.tar.gz
+*
-*.tar.xz
-src/
-pkg/
-sway/