Update libgit2sharp to include libgit2/libgit2sharp#421
Use the new refs API.

Thanks @Saaman!!!!
Manage rcheckin in bare repository
if another commit was done in TFS, keep the repository in good state before reporting error
prevent rcheckin by error in a bare repository
Modify fetch command to manage bare repositories
Manage clone and init in a bare repository
Add branch support in fetch and rcheckin for a bare repository

But should specify the git branch name that should be updated with --bare option

Use :
git tfs fetch --bare=master
git tfs fetch --bare=branch1 -i branch1
git tfs rcheckin --bare=master
git tfs rcheckin --bare=branch1 -i branch1
refactoring of how get ref for a head branch...
corrections....