result" (and similar bug in ADDTIME/SUBTIME).

Both Item_func_add_time/Item_func_timediff::val_str() now use
calc_time_diff() function which was backported from 5.0 (and which
was was fixed to properly handle microseconds part of its second 
argument). Also now we correctly set sign of result in case when
first argument is negative and second is positive.  


mysql-test/r/func_sapdb.result:
  Added test for bug #8068 "TIMEDIFF with first negative argument gives wrong 
  result".
mysql-test/t/func_sapdb.test:
  Added test for bug #8068 "TIMEDIFF with first negative argument gives wrong 
  result".
sql/item_timefunc.cc:
  - Backported calc_time_diff() function from 5.0 tree. Changed it to
    accept time value as its second argument when its first argument
    is datetime value. Fixed wrong handling of microsecond part of
    second argument.
  - Item_func_add_time::val_str()/Item_func_timediff::val_str()
    Removed similar pieces of code calculating difference 
    between two datetime values (or their sum) in microseconds.
    Now we use calc_time_diff() function instead. 
    Also now we correctly set sign of result in case when first 
    argument is negative and second is positive.