dot/shell-rc: Colourize `diff' output similarly to `grep'.

There's a problem here because of a `zsh' bug:

	$ diff -u <(thing) <(thong) | blah
	diff: /proc/self/fd/13: No such file or directory
	diff: /proc/self/fd/14: No such file or directory

caused by the process-substitution pipes being lost somewhere.

Work around this as (a)

	$ diff -u =(thing) =(thong) | blah

(produces temporary files), or (b)

	$ { diff -u <(thing) <(thong) } | blah

(extra typing).