How can I convert .MTS file (AVCHD) to .mp4 by ffmpeg without re-encoding H264 video stream correctly?

Here it is:

ffmpeg -i input.m2ts -c:v copy -c:a aac -strict experimental -b:a 128k output.mp4

This will only copy the video stream without re-encoding and encode the audio track to AAC VBR stereo, it requires a recent FFmpeg version.