~/.zshrc,~/.bin/up: Fast hg branch detection.

It's quite interesting that even `ruby -e ''` and `python -c ''` take
about 0.03s with >90% system load whereas `git branch` is usually at
0.005s and 0% system load.

And `up some-file` is even faster (0.001s w/ 0% system load), which is
understandable as it mostly does just a few directory hops. But still
quite cool that this now works reliably and fast.

I don't know if it's the gc kicking in or if it just takes that long to
load the standard includes/library, but there's definitively room for
mini-optimization because on slower machines it's too much of an
overhead to use in places like the prompt which has to be responsive.
~/.zshrc: Better escaping for the last-cmd prompt.

I think `!` is only used for history expansion, something I don't use
often and it is replaced by the command in question anyway. So that
should be ok?
~/.tmux.conf: Support mouse scrolling.

It's kind of inconvenient because it doesn't allow normal mouse
selection, but we'll see how it works out.
~/.bin/up.rs: A version of up in Rust.

This should be smarter, shorter, sweeter. Or maybe the advantages of
advanced languages are not so apparent in small examples? Except small
examples are what makes them look wonderful. So small applied examples
are no good for them?

Mhh, that's definitively something to play with.