Annotate
========

Broadly tries to ascribe parts of the tree state to individual commits.

There appear to be three basic ways of generating annotations:

If the annotation works by asking the storage layer for successive full texts
then the scaling of this will be proportional to the time to diff throughout
the history of thing being annotated.

If the annotation works by asking the storage layer for successive deltas
within the history of the thing being annotated we believe we can make it scale
broadly proportional to the depth of the tree of revisions of the annotated
object.

If the annotation works by combining cached annotations such that creating a
full text recreates annotations for it then it will scale with the cost of
obtaining that text.

Generally we want our current annotations but it would be nice to be able to do
whitespace annotations and potentially other diff based annotations.

Some things to think about:

 * Perhaps multiparent deltas would allow us to not store the cached
   annotations in each delta without losing performance or accuracy.