templates: fix attach option to create a proper mbox format

There was a bug where the attach option to stg mail was
not spitting out a patch that can be used by git am, because the patch
file only contained the diff.

This is derived from the same fix from Jesse Brandeburg for
the attachinline mbox fix.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
1 file changed