[git] Make prompt display branch name correctly
[git] Remove `emptybranch` alias

Since `v1.7.2`, Git has the ability to create orphan¹ branches, simply
by running: `git checkout --orphan <new_branch>`.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

¹ orphan branch = branch that will have no parents and it will be the
  root of a new history totally disconnected from all the other branches
  and commits

Ref: http://git-scm.com/docs/git-checkout/1.7.2