Submodule libgit2sharp updated to version v0.12.0

Submodule libgit2sharp:
    > 9fd0803 - Remove obsolete members
    > f2e2fac - Release LibGit2Sharp v0.12.0
    > b2e4367 - Update libgit2 binaries to 9d9fff3
    > cc2661b - Enhance Index.RetrieveStatus() test coverage
    > 06a0c57 - Introduce repo.Info.IsShallow
    > 3583599 - Add shallow test repo
    > 48ea4d5 - Generate a unique ID at compile time to work around a CLR bug