Add support of bare repository for init, clone, fetch and rcheckin commands

Use :
//Clone or init a TFS repository in a bare repository
git tfs init --bare [...]
git tfs clone --bare [...]

//Fetch tfs changesets. We should specify the git branch to fetch on because there is no "checkout" branch in a bare repository
git tfs fetch --bare=master
git tfs fetch --bare=nameGitBranch -i nameGitTfsRemote

//Rcheckin commits. We should specify the git branch for the same reasons...
git tfs rcheckin --bare=master
git tfs rcheckin --bare=nameGitBranch -i nameGitTfsRemote

Should work quite well...

PS: to create a bare repository, you could also clone a normal git-tfs repository with the `--mirror` option...