mysql-test/r/func_sapdb.result:
  Added test for bug #7586 "TIMEDIFF for sec+microsec not working
  properly". Corrected previously wrong results of couple other
  statements.
mysql-test/t/func_sapdb.test:
  Added test for bug #7586 "TIMEDIFF for sec+microsec not working
  properly".
sql/item_timefunc.cc:
  Item_func_timediff::val_str():
   Use simplier and less error-prone implementation. Now we are counting 
   difference between time values in microseconds and convert it to time
   value (instead of using seconds and taking difference in "second_part"s 
   into account).