Bach and Hassler:
The popular song was first; text: Mein Gmüt ist mir verwirret von einem Mägdlein zart ... = My mind is bewildered by a tender girl ...
O Haupt voll Blut und Wunden = O head full of blood and wounds is a standard hymn which is sung at Good Friday in almost all services over the country. So Bach had inevitably to use this tune in his Passion.
Especially during Luthers lifetime it was usual to take the tune of a popular song to make a new hymn attractive to the parish. The putting of a hymn to a worldly tune (and vice versa) is called contrafactura.

Wilfried