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

Thanks to Andrew, John and Robert for their feedback on my earlier merge proposal. This updated one takes their comments into account, particularly the bit about optional vs mandatory (hash change vs semantically important) updating. It also drops the "line cache" as there are better ways (e.g. pyrex) or solving serialisation speed (which we can look into later).

As before, this improves 'bzr status' performance on OpenOffice.org by 0.5 seconds after changing a small number of files. As mentioned in NEWS, full status drops from 1.6 to 1.1 seconds while status on just a FILE drops from 1.0 to 0.5 seconds.