append_history(), if libreadline is new enough to have those functions
(they seem to be present at least since 4.2; but libedit may not have them).
This gives significantly saner behavior when two or more sessions overlap in
their use of the history file; although having two sessions exit at just the
same time is still perilous to your history.  The behavior of \s remains
unchanged, ie, overwrite whatever was there.
Per bug #5052 from Marek Wójtowicz.