[RFC] Delete current branch

Hi,

Many of my ideas turn out to be really stupid, and I need to throw
away my feature branch.  So, I find myself doing this often:

  # on branch menuconfig-jk
  $ git checkout master
  $ git branch -D<BACKSAPCE>
  # er, what was the branch name again?
  $ git checkout -
  # Ah
  $ git checkout master
  $ git branch -D menuconfig-jk

So, I scripted it for myself.  Perhaps we should get the functionality
in core as `git branch -Dc` (c for "current"; or something)?

Also, perhaps a `git describe -` corresponding to `git checkout -`?
Then I can use it with --contains --all to get the name of the
previous branch.

Thanks.