Adding in new version of LibGit2Sharp, 0.19.0.

As a result of some changes to the core LibGit2Sharp Library, in order to properly support Stash, I am suggesting that we pull in the latest version of LibGit2Sharp.

This was documented in https://github.com/Particular/GitVersion/issues/246 and then further documented in the equivalent issue in the LibGit2Sharp repository https://github.com/libgit2/libgit2sharp/issues/794.

Big thanks to @nulltoken and @carlosmn for getting the work completed!