You don’t need one. (Probably. I’m just a language lawyer, not a regular lawyer.)
GitHub is a US company, and since you didn’t use the spelling ‘licence’, I assume you are US-based as well, so I’m going to work with the assumption that only US law applies. Under this assumption, it’s probably safe to assume that those metadata are in the public domain, you don’t need a license for them, and you cannot license them to anyone else yourself under copyright law. If you included things like contents of commits or user avatars, those may be potentially copyrightable, but mere names, dates and addresses are not.
The reason for this is that under the US legal tradition, copyright has been applicable only to results of creative effort, and not to mere collections of otherwise uncopyrightable information, however large. The relevant clause of the US Constitution that enables the creation of copyright and patent laws only authorises it to ‘promote the Progress of Science and useful Arts’; a more direct legal justification is the court case Feist v. Rural, which established that mere aggregation of data does not create copyrightability. The US also does not recognise sui generis database rights in a way that some other jurisdictions (especially those in the EU) do.
That said, other kinds of laws and agreements may still be applicable. The Open Database License, for example, attempts to implement copyleft-like provisions under contract law. There are some important differences from copyright law, however: the most important is that you can’t force someone to follow a contract they haven’t agreed to. If you share information with A under a contract, and A shares that information with B, there is little you can do to compel B to respect the contract.
There may also be privacy laws that apply, but which I have not touched upon above at all. Because this data set is based on already-public information, their applicability seems rather doubtful to me, but I would still consult a real expert to be sure.