revlog: abort on attempt to write null revision My repo got corrupted yesterday by something that ended up writing the null revision to the revlog (nullid hash, not nullrev index, of course). We use many extensions internally (narrowhg, remotefilelog, evolve, internal extensions) and treemanifests are on. The null revision was written to the changelog, the root manifest log, and one subdirectory manifest log. I have no idea exactly why the null revision was written, but it seems cheap enough to check that we should fail instead of corrupting the repo. Differential Revision: https://phab.mercurial-scm.org/D522