gifn-apply: add a "reuse" Tox environment to the first stage

We have already prepared the grounds for checking the SPDX
copyright and license tags using the reuse tool; let's invoke it.

Also, actually add the .gitignore file that will help reuse
ignore test artifacts.

Change-Id: Id623c2a2d5d378d1bb427b62b1431d2cbb08a395
diff --git a/tools/git-if-needed/pyproject.toml b/tools/git-if-needed/pyproject.toml
index 894660d..ce37834 100644
--- a/tools/git-if-needed/pyproject.toml
+++ b/tools/git-if-needed/pyproject.toml
@@ -68,4 +68,4 @@
 ]
 
 [tool.test-stages]
-stages = ["ruff and not @manual", "@check and not @manual", "@tests and not @manual"]
+stages = ["@check and @quick and not @manual", "@check and not @manual", "@tests and not @manual"]