Bypass views_field_view with custom handler for item_revision summary

Some commitlog displays on git-dev.drupal.org are taking a long time to render due to an unusually high number of item revisions for each operation, caused by the automated stripping of $Id, or .po/.pot files. Certain pages such as Drupal core can have up to 2500 item_revisions on a single page, and this is causing some major overhead, most of which seems to be in the views render process.

This handler is a terrible hack. It represents all that is unholy about site-specific code, and I'm virtually ashamed to post code that looks like this, but if it makes a big enough difference in the rendering of our commit logs, it may be worth it.

I'm not 100% sure that the markup is correct, and frankly this should probably go in drupalorg instead of Version Control API. Please test this on git-dev and let me know if this is an improvement over the views_field_view approach. (To test, simply remove the Global: View field and add the new summary field.)