Ice cream parlors at the turn of the century used to play minstrel music to keep their customers entertained. A popular song, set to 'Turkey in the Straw', was 'Nigger Love A Watermelon! Ha! Ha! Ha!' - it was very popular in its time.

As a way to make a correlation between the pleasant experiences in a ice cream parlor, early ice cream trucks would also play similar music from music boxes. Leading to the wordless jingling rendition that we all know.

Source: Code Switch article, from NPR

For what it's worth, I've never heard the song before. The standard tune here in Australia seems to be/was "Greensleeves". Any ideas how that happened?