exports: Assert that recently-excluded paths are excluded

Reproduces: https://github.com/flatpak/flatpak/issues/5205
Reproduces: https://github.com/flatpak/flatpak/issues/5207
Signed-off-by: Simon McVittie <smcv@collabora.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f