Unshare history.

I want to be able to airbrush out harsh commands in my history. I
appreciate that zsh can be configured so that it will not record a
command that leads with a space, but if I forget and type something like
`get reset --hard HEAD^` without a leading space, I've now chambered
a round in every prompt on my machine.

Dangerous `git` invocations are especially dangerous, because so often
I'm rerunning a previous `git` command via an incremental reverse search
for `git` in my history. All to easy to press enter too soon or too
late and find yourself a victim of Murphy's Law.

Thus, I don't want to share history, really. It is annoying, actually,
to have the history of a shell where I'm coding get loaded up with a
bunch of commands from a shell where I'm preforming system
administration. My years of bash have taught me to expect that each
shell has it's own recent memory. When I move from one shell to the
next, I can run the history to see where I left off. I might have been
running a command out of history with some variations, and I'm surprised
when an incremental reverse search takes me through a bunch of crufty,
unrelated options.

I don't want to share history, but I do want to airbrush my history.

Disabling SHARED_HISTORY ends shared history and makes airbrushing work
the way it worked for me in bash.

Now, I do actually want a record of things that I've done, in case I'm
wondering how long I've been working at something, or I want to know the
exact time I ran a command for the sake of understanding some unexpected
outcomes. That is why I've enabled INC_APPEND_HISTORY, which will append
each command to the history file as it happens. This creates a record of
my time in the shell, an history if you will.

If it turns out that I want to share history, I can run `fc -R` to load
the history file, so I still get to share history, but only if that is
what I really want.

I probably don't. I use GNU Screen to keep a dozen or so shells open,
and work a little bit in each one. I run commands out of history
frequently, so each shell has a history that acts as short-term memory
for that shell. I don't see how history across a dozen shells does
anything but detract from the utility of history.