Target branch can't be second argument to merge

Remove instances in tutorial where the target branch for a merge is
supplied as a second argument to `git merge`.  The target branch for
merge is always the current branch.
No multiple arguments for merge

`git merge` should only be given multiple arguments if doing an octopus
merge which isn't currently supported.  Require that exactly one
argument be given, and always use HEAD as the currentLocation.
Merge pull request #49 from aschrab/merge-args

Fix merge argument handling