Add two small shell utilities to generate and compare diffs between
edid-decode and di-edid-decode. Store the diffs in-tree and add tests
to ensure they don't regress. Run the tests in CI with a pinned
installation of edid-decode.

One EDID blob is added to the test collection: dell-2408wfp-dp. It's
extracted [1] from the edid-decode repository.

[1]: https://git.linuxtv.org/edid-decode.git/tree/data/dell-2408wfp-dp



Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: emersion/libdisplay-info#5