Allow access to the cached peel data in packed-refs

The `packed-refs` file already contains peel data for Hard Tags, stored in a newline after the given commit and preceded by `^`. We're currently not making use of this peel data anywhere. This PR shifts things around a little bit so we can store the peel OID in `git_reference`s and make it available through an API call, while also speeding up `git_reference_peel` to use this information.