For quite some time I have been using Meld to detect relevant changes between two releases when I update a package in Gentoo. I run

# ebuild foobar-1.2.ebuild manifest prepare
# ebuild foobar-1.3.ebuild manifest prepare

and throw Meld at both outputs

# meld /var/tmp/portage/[..]/foobar-1.2/work/foobar-1.2/ \
       /var/tmp/portage/[..]/foobar-1.3/work/foobar-1.3/ &

Meld makes it easy to see what has changed and (especially) what has not changed. With sole diff -r that would be difficult. I usually start by inspecting changes to configure.ac. If upstream did a good job that diff tells what dependencies to touch, already. Near the left and right margin you can see where else the file has been modified. No need to scroll-search down for more: you already know what you get.

The NEWS and ChangeLog files usually offer pointers of interest, too.

If my hint on Meld made a single Gentoo packager juggler's life easier or more efficient, I have achieved what I was aiming for. Sorry for the noise to everyone else.

PS: The second preview image up there has been losslessly reduced by 40% in size just by running it through optipng -o7.