Code review comment for lp:~bzr/bzr/faster-commit-file

This is effectively a resubmission of my earlier patch 6 weeks ago, with some minor clean-ups. While I agree that we can do much better than this by pushing more functionality down into iter_changes, I still believe this is a safe and valuable step forward. For example, usertest results (run tonight) on the 'selective commit' task comparing bzr.dev r4476 to r4476+patch shows an improvement from 73.4 seconds to 3.0 seconds for OOo on format 2a. In other words, this patch improves performance from "absolutely terrible on large projects" to "acceptable for most users".

FWIW, I did spent quite a bit of time over two weeks trying a different approach, namely smarter use of iter_changes - doing include filtering there - plus decorators over those results. That proved to be a dead-end because of safety concerns: race-conditions around dirstate locking mean wrong results are possible. In comparison, this patch is safe as best I can tell.

I'll leave it to others to decide whether this is enough progress on this problem for us to ship 2.0. At a minimum, I think it changes the bug priority from Critical to something less.