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

This patch speeds up 'bzr log FILE' on flat-ish histories, as commonly found after an import from svn, cvs and other central VCS tools. On OOo, it drops the time taken down from 29 seconds to 1.5 seconds for logging a typical file.

The key to this improvement is starting with the per-file graph and searching the mainline until the revisions of interest are found. That works very well when the history of a project is flat or mostly flat, because it avoids the 27 seconds required to calculate the full revision graph. In a nutshell, the algorithm changes from O(full-history) to O(file-life).

There's certainly room for further smarts here but this is a useful step forward as it stands I feel.