Actually, the problem is messier than I thought. Under some circumstances,
the missing rev in .hgsubstate makes it completely impossible to run "hg
update" or "hg update -C", so I have to manually whack things in this manner:

hg debugsetparents tip
hg debugrebuildstate
hg cat -rtip .hgsubstate > .hgsubstate
vi .hgsubstate
hg ci -m "Barf-o-rama" .hgsubstate

That's really nasty.