Command to delete merged branches

This is handy after a bunch of your local branches get merged via pull requests.
Judging by the discussions [here](http://stackoverflow.com/questions/669452/is-preferable-over-in-bash-scripts/669486#669486), any bash release since the the mid-90s should support it. Not sure how much you care about running your git helpers on embedded Solaris devices from 1997...