delay progress until Release files are downloaded

Progress reporting used an "upper bound" on files we might get, expect
that this wasn't correct in case pdiff entered the picture. So instead
of calculating a value which is perhaps incorrect, we just accept that
we can't tell how many files we are going to download and just keep at
0% until we know. Additionally, if we have pdiffs we wait until we got
these (sub)index files, too.

That could all be done better by downloading all Release files first and
planing with them in hand accordingly, but one step at a time.

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