Before running a command, we add the command to history, so
that if the command causes us to exit it's still captured in
history. But that command should not be considered part of
history when expanding the history within the command itself.
For example, `echo $history[1]` should be the previously
run command, not `echo $history[1]` itself.
Fixes#2028
U+F8FF is the last character in the private use area, but it's also the
codepoint used for the Apple symbol (), which is typeable on US
keyboards in OS X, and so should actually work.
