lib/ignore: Don't crash in partial #include line (ref #5985) (#5986)

If the line is just "#include" with nothing following it we would crash
with an index out of bounds error. Now it's a little more careful.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f