RFC: Flat directory for notes, or fan-out? Both!

Hi,

Shawn triggered some well needed thinking on my part about the notes implementation. At the moment, we have flat directory structure, and read all of them in one go (when needed).

I think we should support that, because it is relatively easy to generate that kind of trees for small-scale applications.

However, I think there is also a benefit to handle fan-out directory structures, too: they scale much nicer.

If the commit name was not found as a filename, it could be searched in whatever subdirectory whose name is a prefix of said commit name (first wins).

So I think it would be a sane plan to do the following when a commit note is requested:

- If not done yet, read in the whole top-level directory of the notes ref.

- If the commit name is not found, find the tree entries whose name is a 
  prefix of the commit name (we can even use the same hashmap to store 
  these "incomplete" names, as we use a linear hash, which we fill in 
  ascending order),

  - read the trees one by one, until the commit name is found (or no tree 
    entry is left), deleting the trees from the hashmap on the go.

How does that sound?

Ciao, Dscho