bashrc: fix reference to /dev/null

I'd previously missed the leading slash to /dev.
git config: change diff.new colour

With my current shell colour scheme, I prefer having the new lines of a
diff colours cyan rather than green.
gitk: update

I haven't kept my version-controlled copy of `gitk` up-to-date with my
working copy of it.
bashrc: add to PATH for zinc and DEVARCH

Needed for doing DEVARCH development.