the git-new-workdir helper is mostly useful to have one pristine clone
where the updates get pulled into & from that you create working copies.
As pulling to that master copy only updates the current branch (usually
master), checking out a different branch using git-new-workdir will
result in "your branch is behind the remote branch by xx commits" when
there have been commits after the branch was first checked out locally.
Thus this change adds a fast-forwarding merge when a branch is given.
