Do not store .diff_Index files in update

Nowadays we only download the index file if we have a non-current file
on disk which we want to patch. If that is the case, any index file for
patches we could have stored is by definition outdated, so storing those
files just takes up disk space.

At least, that is the case if we have a Release file – if we don't this
commit introduces a needless redownload for such repositories but such
repositories are an error by default and if they can't be bothered to
provide a Release file its very unlikely they actually ship diffs, so
adding detection code for this seems pointless at best.

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