Here's my patch that implements the behavior that skips the history
rewrite-after-appending if zsh is exiting because it received a signal.
This should make the simultaneous closing of multiple zsh programs less
contentious over the history file.  The safest way to go is to use
INC_APPEND_HISTORY or SHARE_HISTORY (since new history lines are being
appended as they are entered, not all at once on exit).

People who don't append to the history file won't see any change in
behavior from this patch (that will have to be improved in some other
manner, possibly by switching over to using a *.new file to write out
the new history lines).

..wayne..