Scratch the git_revp_attr configuration of a git_revp

Scratch the git_revp_attr configuration of a git_revp

This isn't the best idea I've head.  Pierre Habouzit was suggesting
a technique of assigning a unique integer to each commit and then
allocating storage out of auxiliary pools, using the commit's unique
integer to index into any auxiliary pool in constant time.  This way
both applications and the library can efficiently attach arbitrary
data onto a commit, such as rewritten parents, or flags, and have
them disconnected from the main object hash table.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>