Code review comment for lp:~ian-clatworthy/bzr/faster-dirstate-saving

This updated merge proposal makes it configurable as to how many changes are ok before the dirstate should be saved. As Martin requested, there's the ability to turn off saving altogether. After this lands, we can use that to implement various policies (e.g. don't save on a logical read-only operation) as we see fit.

This drops 'bzr st [file]' down by 0.5+ seconds on OOo. Nice. It doesn't yet make a difference for add or commit. Those are small follow-on patches after we're happy with this one.