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.
Merge pull request #248 from gep13/LibGit2Sharp0.19.0

Adding in new version of LibGit2Sharp, 0.19.0.