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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