Recently I turned on next-error-follow-minor-mode in diff buffers, so
the source context is shown automatically as I move around in the diff
buffer.

It works well, but after a while I noticed there is always a 'Hunk
already applied' message in the echo area.

I investigated and turns out the message comes from here:

    next-error
    next-error-function
    diff-next-error
    diff-goto-source
    diff-hunk-status-msg


So goto source prints this message every time, even if one just
presses RET in a diff buffer.

I found it confusing, because I was just moving around, I didn't try
to apply anything, so this message wasn't really informative, it
stopped my flow with its unwanted appearance and reference to an other
operation.

Shouldn't this 'Hunk already applied' message appear only if the user
actually tries to apply something?