revlog: add exception when linkrev == nullrev When we deployed the latest crew mercurial to our users, a few of them had issues where a filelog would have an entry with a -1 linkrev. This caused operations like rebase and amend to create a bundle containing the entire repository, which took a long time. I don't know what the issue is, but adding this check should prevent repos from getting in this state, and should help us pinpoint the issue next time it happens.