remove verboseness of path outputs

[plastikfan]

Seeing output lke this:

dummy:executing: 'magick /Users/plastikfan/dev/test/pics/blur/TRASH/blur/TRASH/blur/TRASH/screen-shot-1.png --gaussian-blur 0.05 --interlace plane --strip /Users/plastikfan/dev/test/pics/blur/TRASH/screen-shot-1.png'

is too verbose. The full paths should be shown in the log, but the output should contains only relative paths, to reduce the output the user has to contend with.

And to simplify more, consider not outputing for every file, just write a summary.

make use of filepath.Rel