When the contents of .hgsubstate are stale (either because they've
manually been tweaked or partial updates have confused it), we get
confused about whether it actually needs committing.
So instead, we actively consult the parent's substate and compare it
the actual current state when deciding whether it needs committing.
Side effect: lots of "committing subrepo" messages that didn't
correspond with real commits disappear.
This change is fairly invasive for a fairly obscure condition, so it's
kept on the default branch.
