Add reverse indexes for packfiles

Go-git has recently introduced support for reverse indexes, both for
encoding and decoding. In order to expand on tests and start relying
on the reverse indexes, we need to have git-generated .rev files so
that we can better test go-git.

All these files were generated using:
git index-pack --rev-index -v <packfile>

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

Author: pjbgf