[boost] [git] Creating merge bases.

Hi, After creating a merge base for iostreams last night, I thought I might do it for some other modules where I can find a good merge base. I won't do it for any modules with post-subversion commits on master, and I won't change any content without a pull request, so this shouldn't be disruptive. Does this sound okay? It should help anyone wishing to work on a module, as they can get a better idea of what hasn't been merged to master, and it should make it easier to merge changes. Daniel