cats: fixes BigSqlQuery header fetching

[joergsteffens]

BigSqlQuery fetches data in 100 row chunks.
This change makes sure, that subsequent chunks
also have valid header information.

Fixes: https://bugs.bareos.org/view.php?id=1007 JSON API: 'list files' returns malformed json

OP#5753

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[sebsura]

Great work! I requested some changes below. I think we can also easily add a test for this.

Having the tmp/file-list contain tmp/data twice is enough to get over the 100 file mark. Afterwards we can check if a list files outputs the correct headers.

[sebsura]

I added some feedback to your test.

[sebsura]

Thanks for your work. I added some comments. Feel free to ignore them.