Git script that fast forward merges and deletes the old branch
The order of the arguments is backwards, I think. Compare against git-rebase(1):


  Synopsis
  
  git rebase [&lt;upstream&gt;] [&lt;branch&gt;]
  
  Description
  
  If &lt;branch&gt; is specified, git rebase will perform an automatic git checkout &lt;branch&gt; before doing anything else. Otherwise it remains on the current branch.


The git-rebase order is also better since the git checkout can be considered optional.  You should probably make yours optional as well.

The help message needs to be much more informative about what the command does and how to use it. Git is hard enough to use as it is.

Accidental branch deletion is unlikely to be tragic, since you used -d and not -D.