PackExt: Add a reverse index extension.

PackExt: Add a reverse index extension.

There is no reverse index PackExt because the reverse index is not currently
written to a file. This prevents fine-grained performance reporting for reverse
indexes, which will be useful when introducing a reverse index file and
observing performance changes.

Add a reverse index extension that matches the one in cgit
(https://github.com/git/git/blob/9bf691b78cf906751e65d65ba0c6ffdcd9a5a12c/Documentation/gitformat-pack.txt#L302)
in preparation for adding a reverse index file while observing
performance before and after.

Change-Id: Iee53f1e01cf645a3c468892fcf97c8444f9a784a
Signed-off-by: Anna Papitto <annapapitto@google.com>