Reported by Thomas Keller, Oct 4, 2010

For some tags the "branches" stanza of automate tags keeps 
empty, for example when the branch certificates for the specific 
revisions are not pulled (and maybe also when the branches in 
question are suspended, but I haven't tested this).

The issue is here if this is valid basic_io syntax at all or if we 
shouldn't simply skip the "branches" stanza altogether 
when we have no branches to output...? The docs already note that 
the list is "possibly empty", but it still looks a bit 
fugly.