lint: ignore files ignored by git in mlc

Updating to MLC v0.18.0 includes a new feature which will ignore all
files ignored by git: `--gitignore`.

This helps avoid false-positives flagged by this linter in non-project
files, such as a developer might expect to have in their directory (e.g.
guix-builds, python venvs, etc.)

test/lint/test_runner/src/main.rs

@@ -410,6 +410,7 @@ fn lint_markdown() -> LintResult {
         "--offline",
         "--ignore-path",
         md_ignore_path_str.as_str(),
+        "--gitignore",
         "--root-dir",
         ".",
     ])