Make PackReverseIndex a public structure

Repository inspection tools may find building a reverse index on a
pack useful, as they can then locate an object by offset. As both
C Git and JGit sometimes produce error messages with the offset
rather than the SHA-1, it may be useful to expose this type.

Change-Id: I487bf32e85a8985cf8ab382d4c82fcbe1fc7da6c
1 file changed