hg2git: start incremental conversion from a known commit Previously, we'd spin up the Mercurial incremental exporter from the null commit and build up state from there. This meant that for the first exported commit, we'd have to read all the files in that commit and compute Git blobs and trees based on that. The current Mercurial to Git conversion scheme makes most sense with Mercurial's current default storage format, where manifests are diffed against the numerically previous revision. At some point in the future, the default will switch to generaldelta, where manifests would be diffed against one of their parents. In that world it might make more sense to have a stateless exporter that diffed each commit against its generaldelta parent and calculated dirty trees based on that instead. However, more experiments need to be done to see what export scheme is best. For a repo with around 50,000 files, this brings down an incremental 'hg gexport' of one commit from 18 seconds with a hot file cache (and tens of minutes with a cold one) to around 2 seconds with a hot file cache.