libgit2: 0.27.8 -> 0.28.4
* https://github.com/libgit2/libgit2/releases/tag/v0.28.0
* https://github.com/libgit2/libgit2/releases/tag/v0.28.1
* https://github.com/libgit2/libgit2/releases/tag/v0.28.2
* https://github.com/libgit2/libgit2/releases/tag/v0.28.3
* https://github.com/libgit2/libgit2/releases/tag/v0.28.4