Update libgit2sharp to v0.13
Better warning message if no root branches found
GitTfsTest : Correct accordingly to the new way to setup a git repository
Merge pull request #418 from pmiossec/better_clone_msg

Better warning message if no root branches found
Fix (?) the travis build.
Merge pull request #417 from pmiossec/update_libgit2sharp

Update libgit2sharp to v0.13