Code review comment for lp:~jameinel/bzr/2.1.0b4-convert-kg-heads

See this graph:
http://launchpadlibrarian.net/36247556/conversion_times.png

For the net effect of this patch. In converting mysql from 1.9 => 2a it cuts the time for the conversion approx 2:1.

The basic observation was that we spend *way* too much time graph walking during the conversion. Part of that was because we didn't pass *any* graph object down, so we didn't have any benefit of caching, etc. So for all heads() requests we paid the full api layering (translating back and forth from strings <=> tuples for all entries in the history, etc.)

Anyway, we have a very fast heads implementation, so lets use it. I put in the check:

if ... len(revision_ids) > 100:

in there, so that we don't *always* load the full history for small conversion updates. However, for anything more than that, we probably get a net win out of loading the whole ancestry and using the faster heads implementation.

I wish there was a way I could put the image inline, like I used to be able to do with Thunderbird sending requests via Bundle Buggy. (At least, people using email clients would see the image right away...)