[RFC/PATCHv2 0/6] generation numbers for faster traversals

Here's an updated version of the series posted here:

  http://article.gmane.org/gmane.comp.version-control.git/176861

I'll discuss specific changes in each patch, but the summary is:

  1. object-cache is now called metadata-cache.

  2. The interface to "decorate" and "metadata-cache" is a little
     cleaner, and it's harder to break it by changing the "width" field
     at runtime.

  3. Cache files have a header with a version for future-proofing.

  4. Cache files on disk are automatically invalidated if grafts or
     replace refs change.

The patches are:

  [1/6]: decorate: allow storing values instead of pointers
  [2/6]: add metadata-cache infrastructure
  [3/6]: commit: add commit_generation function
  [4/6]: pretty: support %G to show the generation number of a commit
  [5/6]: check commit generation cache validity against grafts
  [6/6]: limit "contains" traversals based on commit generation

-Peff