revlog: fix revlogio.packentry corner case
We want to store version information about the revlog in the first
entry of its index. The code in packentry was using some heuristics
to detect whether this was the first entry, but these heuristics could
fail in some cases (e.g. rev 0 was empty; rev 1 descends directly from
the nullid and is stored as a delta).
We now give the revision number to packentry to avoid heuristics.
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