mbox: Get better at guessing cover letter's timestamp
Use the first patch timestamp if available, otherwise use series
completion date.
Signed-off-by: 
Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Use the first patch timestamp if available, otherwise use series
completion date.
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>