Update libgit2 to 4eb97ef

https://github.com/libgit2/libgit2/compare/3f8d005a82b39c504220d65b6a6aa696c3b1a9c4...4eb97ef3bf18403fbce351ae4cac673655d2886a
Let's stop this dance. How about adding a dedicated .targets (à la **[Kudu](https://github.com/projectkudu/kudu/blob/bb1e4bd69ccbd86268888aec88316f13f50b7108/Build/Kudu.tests.targets)**)?