The history pack writer had a bug where if the same node was added to the
mutablehistorypack N times, it would write out that it had N entries, but then
it would only write a single entry. This caused corruption (the length value
didn't match the actual number of entries) that broke repack.
This primarily affected users who used the old version of treemanifest (where
trees were converted on the client side). The new version of treemanifest only
seems to repro this in rare cases, like when rebasing multiple commits that
create the same trees.