support for non-commit object caching in rev-cache

support for non-commit object caching in rev-cache

Summarized, this third patch contains:
 - support for non-commit object caching
 - expansion of porcelain to accomodate non-commit objects
 - appropriate tests

Objects are stored relative to the commit in which they were introduced --
commits are 'diffed' against their parents.  This will eliminate the need for
tree recursion in cached commits (significantly reducing I/O), and potentially
be useful to external applications.

Signed-off-by: Nick Edelen <sirnot@gmail.com>