administrative functions for rev-cache, start of integration into git

administrative functions for rev-cache, start of integration into git

This patch, fourth, contains miscellaneous (maintenance) features:
 - support for cache slice fusion, index regeneration and object size caching
 - non-commit object generation refactored
 - porcelain updated to support feature additions

The beginnings of integration into git are present in this patch, mainly
centered on caching object size; the object generation is refactored to more
elegantly exploit this.  Fusion allows smaller (incremental) slices to be
coagulated into a larger slice, reducing overhead, while index regeneration
enables repair or cleaning of the cache index.

Note that tests for these features are included in the following patch, as they
take advantage of the rev-cache's integration into the revision walker.

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