GraphicMagick branched off ImageMagick's version 5.5.2 in 2002 after
irreconcilable differences emerged in the developers' group.[1]
Some systems might have GraphicMagick and diff-image works just fine with
it, so add a few lines to automatically support it as well, but expect
users to install the compatibility package that adds the symlinks for
commands 'compare' and 'montage'.
[1] https://en.wikipedia.org/wiki/GraphicsMagickCloses: #37
in the EXIF info.
This means that we won't run the (potentially expensive) first compare if we
have seen a metadata change, in particular a file size change. We already
know that there's a difference.
