deal with partially downloaded changelogs

Changelogs are relatively small and we have no hashes for them, but we
had partial support for them before, so lets stick to it.

This also deletes the (partial) file before moving the downloaded file
into its place – rename(2) should be doing this by itself, but testing
on semaphoreci suggests that this isn't always the case (error is "Stale
file handle") and we don't need an atomic replace here, so be explicit.

Git-Dch: Ignore

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