Code review comment for lp:~ian-clatworthy/bzr/faster-log-file

This patch speeds up log FILE on flat-ish histories: 29 secs => 1 sec for OOo. As noted in the previous merge proposal, it does this by starting from the per-file graph and looking for the revisions of interest along the mainline, instead of always assuming that the full revision graph is required. This win is achieved for files where the history is flat or mostly flat during the time between last edit and creation.

In response to John's feedback on the earlier proposal, this version now has some additional checks to ensure that we don't walk the whole mainline only to come up short. In particular, it uses revision timestamps as a sanity check every now and then. It also bails out if the revision tree is clearly dense during the file's edit life-time. So even as flat histories mutate into dense histories, the win will remain for most files.

Collectively, these new checks maintain the original win while keeping the overhead on dense trees to a small enough (IMO) amount: ~ 10%. For example, 'bzr log NEWS' on Bazaar's trunk goes from 2.7 to 3.0 seconds while 'bzr log bzrlib/builtins.py' goes from 2.4 to 2.6 seconds. That 0.2-0.3 second increase isn't noticeable in practice, while the reduction from 29 to 1 second obviously is.