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

context: remove unnecessary default values for matchers (API) ctx._dirstatestatus() is called only from workingctx._buildstatus() and that function, in turn, is called only from basectx.status(). basectx.status() will always pass a matcher to _buildstatus(), so there's no need to handle a None matcher there. Differential Revision: https://phab.mercurial-scm.org/D492

context: always pass a matcher into _matchstatus() (API) This just makes it a little easier to follow and removes the need to call the superclass's method in workingctx. Differential Revision: https://phab.mercurial-scm.org/D491

test-casecollision-merge: fix the test D30 (055fee3547df) makes the error message unpredictable, therefore the fix. Differential Revision: https://phab.mercurial-scm.org/D494