Bash: undo $PS1 \W workaround since bug fixed

The latest homebrew version of Bash no longer crashes when you have \W
or \w in your $PS1/$PS2/etc. and `cd` to certain directories (like /).
We can therefore now safely use \W in our prompt, and delete the
workarounds we were using previously.
Bash: fix path to homebrew autoupdate script

We previously moved this script from ~/bin to ~/mac-scripts. We also
moved the code for displaying the 'please update homebrew' banner from
.bash_profile to this script. Because we had the incorrect path, that
script was never being run at login and we weren't ever seeing the
update banner. This fixes that bug.
Git: Create wrapper to fix Kaleidoscope bug

There is a bug with the current version of Kaleidoscope and git where,
if you launch Kaleidoscope from the terminal using the ksdiff utility,
it will crash when trying to launch git as a subprocess. This doesn't
appear to have any real effect, but it causes annoying OS X error
messages to pop up.

This script acts as a wrapper to ksdiff. It first checks if
Kaleidoscope is open and, if not, will launch it from a new environment
which doesn't inherit settings from the current one (I believe the
error comes from certain variables being set in my highly customized
Bash environment, which Kaleidoscope can't deal with.) It then pauses a
second before running ksdiff as normal, which re-uses the existing
Kaleidoscope process, avoiding launching a new one tainted by this
terminal.

Hopefully I'll be able to delete this wrapper when Kaleidoscope fixes
the bug.