What do y'all think about this? It seems to be common to have a buffer that's modified (perhaps on disk, perhaps not), and to want to annotate it for display.  This call would allow re-use of the expensive part of the blame, and just adding the buffer-to-newest-commit diff on top of it. The results would need a private backing store that has all the pieces needed to do that last diff.

/cc @kevinsawicki