If you really must attend a holiday concert, make it something musicologically interesting. In what has become an annual tradition (see the 2005 and 2006 installments), the Folger Consort is presenting the most appealing and satisfying Christmas concert in the city. More than just a concert, it is a staged production of the Second Shepherds’ Play, an English mystery play from the Towneley cycle. Director Mary Hall Surface began by modernizing the play’s Middle English…