[PATCH 0 of 2] mq: handle substate modifications safely

[Kevin Bullock]

This series reverses course from the approach taken in
<http://selenic.com/repo/hg/rev/f08df4d38442>, allowing MQ to modify the
substate as long as the subrepos themselves aren't dirty.

In other words, the substate of the outer repo can be modified by
patches to just the outer repo, as long as no commits within the
subrepos are needed. This means the substate can be managed by MQ.