Upgrade binaries to current LibGit2 HEAD. (5d91bea)

DO NOT MERGE YET.

Okay so this is the bump itself. I split it into binary update then API usage update.

I have a few things to mention:
44 unit tests fail, mostly due to current Oid being set to 0 (old one being correct) in Diff/Status. I'm unsure what causes the files to appear as modified. It might be a bug in libgit2? I'll keep checking it out.

This new MergePreference thing should be exposed, right? So, how do I handle it? Move the result to a ref parameter or make the return value a Tuple/KeyValuePair?

A few things which I will handle later on:
* There are now callbacks for ssh_interactive
* There is new remote API and diff stats API.
* There is a config snapshot API.
* There is cherry-pick API.