Note that aborting in subrepo.state() prevents "repairing" commands like revert
to be issued. The user will have to edit the .hgsubstate manually (but he
probably had already otherwise this would not be failing). The same behaviour
already happens with invalid .hgsub entries.
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.
