You mean you don't like the full 40-character-hash?  ;-)

I actually thought of that since the full 40-character hash is stored in the tar file anyway; the issue however
is that GitHub doesn't care if you provide the short value or not.  The file you get will be named with the full
40-character hash.  Since that is the case, I left it using the %{commit}.  Now, if you want to rename the
file being downloaded, you can do that by appending #/$newname as shown in the Bitbucket example.