I hope to one day push the highlighting inside of git itself. That should make it more efficient, and we can do a true LCS diff to find the common parts, rather than the front/back heuristic it currently does (see the diff-highlight README for the gory details).
It's just a little complicated because of the way it interacts with existing internal diff features (in particular the whitespace-error highlighting).
Guile may be the closest, but it failed to gain much traction despite support in some popular GNU tools like GDB. It's certainly not installed on most systems, but it at least has a tiny chance of being present.
On Arch Linux it's a dependency of both gdb and make, but it seems that other distributions go out of their way to avoid shipping it (Debian e.g. builds gdb without a hard dependency on it, and you need to install a separate make-guile package to get guile-enabled make).
Is guile part of the default packages on any unix-alike distribution though?
The huge upside of perl is not necessarily that it's available, but that it's available by default.
Even more OT, but: This script is not new at all: https://github.com/git/git/commits/master/contrib/diff-highl...
The new diff script (https://github.com/git/git/blob/master/contrib/diff-highligh...) is juicy indeed.
It is written in Perl. The Perl interpreter is available on most UNIX systems, so that's handy. I wonder if there is any Lisp-like language that is as common, that you can write scripts and expect them to run on most systems. Would be nice since Lisp languages have very few concepts to learn...