Make GTReflog's initWithReference public

[Uncommon]

-[GTRefLog initWithReference:] is in GTReflog+Private.h for reasons that are not clear to me. I've found a situation where I want to use it, so I ended up writing a category interface to expose it since I can't access that header from the framework.

My situation is that I want to get information about a specific stash, so I'm imitating what's in git_stash_foreach, except using ObjGit classes and using a specific index instead of iterating over all stashes.

I'll gladly do a pull request, but I wanted to make sure there isn't an alternative that I missed.

[pietbrauer]

I don't see a reason why it should be private, would be happy if you would do a PullRequest.

[pietbrauer]

Closed via #596