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.