[_qxjp]

Update: here's a one-liner that will do exactly that

ffmpeg -i input.mp4 -strict -2 -crf 51 output.mp4 && rm input.mp4 && mv output.mp4 input.mp4

For better results, run the above command ten times:

for ((n=0;n<10;n++)); do ffmpeg -i input.mp4 -strict -2 -crf 51 output.mp4 && rm input.mp4 && mv output.mp4 input.mp4; done