ckeditor::diff::tests spawns `git init` / `git config` inside a
tempfile::tempdir so each test has its own self-contained repo. Run
directly via `cargo test --workspace` that works, but under the
pre-commit hook git inherits GIT_DIR (and GIT_WORK_TREE,
GIT_INDEX_FILE, GIT_COMMON_DIR, GIT_OBJECT_DIRECTORY) from the hook's
environment. Every git invocation then secretly operates on the outer
repo's .git rather than the tempdir, so two parallel tests race on
.git/config.lock and one of them quietly stages a `seed.txt` into the
outer repo's index.

The git test helper now scrubs those env vars per Command, so the
tempdir's local repo is the only thing each git invocation sees.
Tests now pass under both `cargo test --workspace` and the pre-commit
hook environment.

@@ -293,0 +375,4 @@
+    }
+}
+
+fn paint_tab_background(