Running `touch` on a tracked file in Git, to update its timestamp, and
then running `meson dist` would cause dist to fail:
ERROR: Repository has uncommitted changes that will not be included in the dist tarball
Use --allow-dirty to ignore the warning and proceed anyway
Unlike `git status` and `git diff`, `git diff-index` doesn't refresh the
index before comparing, so stat changes are assumed to imply content
changes. Run `git update-index -q --refresh` first to refresh the index.
Fixes: #12985