Show merge proposal links in cgit

It would be useful for merge commits displayed in cgit (i.e. git.launchpad.net) to link back to the Launchpad merge proposal associated with them, where one exists.

From the cgit point of view I think this is doable with the "commit-filter" interface, although optimising that would probably require some fun such as writing a Lua script on the fly.

We may need new or better-optimised Launchpad APIs to extract the necessary information quickly enough. I haven't looked into this.