Fix 'filter --shell' command not being invoked for commits

The shell command provided to filter --shell is never invoked for commits because the attributes set it passes to data_traverse() is unconditionally empty, so data_traverse() never even attempts to apply the filter to any fields of the commit.

This was broken by 2014-04-17T07:07:49-04:00!esr@thyrsus.com, which also broke filter --dedos due to the same cause, however, --dedos was fixed by 2014-04-17T07:14:25-04:00!esr@thyrsus.com, but --shell never was, so fix it now.