Between 1965 and 1969 the harpsichord lit up recordings by everyone from Miles Davis to Brigitte Bardot, the Beatles to the Four Tops, Elton John to Waylon Jennings. But what was it about this boxy, donkeyish uncle of a piano that could transcend such disparate contexts? And what made it such a ubiquitous presence internationally in the music of the 1960s? By Tom Greenwood