stg mail should use patch numbering for a single patch if a cover note is specified

[dhowells]

If "stg mail" is given a single patch and a cover letter to send, it doesn't insert the usual patch numbering into the subject fields; however, I would expect the cover to be 00/01 and the patch to be 01/01. Can it be made to do this - or at least take a config option to make this the default?