MADV_MERGEABLE for history

[Matt Corallo]

I tend to use bash_history as a way to remember specific commands to run for various tasks, often 
quite irregularly, and thus keep HISTFILESIZE quite large (800k lines in my .bash_history, for 
32MiB). bash/readline handle this just fine, except that I often have something like 50 instances of 
bash running in various terminal emulators. This leads to something like 7.5GB of memory dedicated 
to bash instances (around 150MB per instance), with most of it going to history storage.

While various optimizations to improve the space efficiency of history would be welcome, ultimately 
this really feels like a task for Linux's samepage merging logic. If the history strings themselves 
were put into contiguous memory (okay, not incredibly trivial, but arenas aren't so complicated), it 
should be fairly straightforward to just `madvise(addr, length, MADV_MERGEABLE)` and let KSM handle 
the rest.

I can of course set a lower HISTSIZE than HISTFILESIZE and set histappend to only load some of 
history in memory, but even the last 10k lines of history leaves me often grep'ing bash_history 
manually. Maybe I'm missing some obvious way to reduce redundant many-bash-instance memory usage, 
though?

Thanks,
Matt