When an old line and a new line are very similar, it's hard to immediately see the difference, so highlighting the changed parts might help.
What you need to do are just downloading a perl script and configuring your git.
$ curl -O https://raw.github.com/git/git/master/contrib/diff-highlight/diff-highlight
Move the file diff-highlight to the ~/bin/ directory (or wherever your $PATH is), and then add the following to your ~/.gitconfig
[pager]
log = diff-highlight | less