Add Commit::getRefs() (for Commits returned from Repository::getLog())

[clue]

First off, thanks for providing this really great library, it really works like a charm. I was looking for a way to create a graph (https://github.com/clue/graph) including all branch labels from my commit history and stumbled upon a couple of issues using this library.

It would be nice if branch labels would be exposed via Commit::getRefs(). Using git log --decorate=full includes all local and remote branches and tag names in the commit log, so that's probably a good start.

[matteosister]

many thanks! ☺️
--decorate=full is really great. I'm gonna dig into it ASAP.