Without this, the rewriteutil logic thinks it's found a reference to the wdir
pseudo-revision, and then tries to look it up and rewrite it. Stop it from
doing that.
Amusingly, I had trouble working with this changeset when I didn't
describe the defect above using a regular expression, because it would
trigger the bug in my installed version of hg.
I can't test because I lack easy access to a filesystem with execbit support, but will 0{6,64} also fail?
Also, I didn't think about this when I swiped this from evolve, but should we be locking the repo before doing anything? Otherwise, the lookup here could succeed, but the unfi[fullnode] below could fail.