Since we added the use of git submodules recently, this trivial script
has been helpful to me to "git clean" not only the top-level Open MPI
repo, but also all the included submodules, too.
NOTE: this script does the (harsh) "git clean -dfx" command, which
deletes everything that git does not know about. Use with care!
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
