Update libgit2 to c5cacc4

This updates ```libgit2``` to libgit2/libgit2@c5cacc4ec2c13e93164b8e3013b0d18c8c6d820c. I've contributed some code to get ```git cherry-pick``` and ```git format-patch``` to work.

This update allows for those PR's to be created. Looks like ```git subtree``` didn't mess this one up.