Update libgit2 to 4eb97ef

https://github.com/libgit2/libgit2/compare/3f8d005a82b39c504220d65b6a6aa696c3b1a9c4...4eb97ef3bf18403fbce351ae4cac673655d2886a
That's clever.  I'd like to address this in a separate PR, if you don't mind, but I think it's a wonderful idea.