The mirror logic assumes that it has a mirrors list with at least once
valid entry in it. In the index format as defined by `fdroid update`,
there is always at least one valid URL: the canonical URL. That also
means if there is only one item in the mirrors list, there are no
other URLs to try.
The initial state of the repos in the database also include the canonical
URL in the mirrors list so the mirror logic works on the first index
update. That makes it possible to do the first index update via SD Card
or USB OTG drive.
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