Chris Willman Senior Music Writer and Chief Music CriticNews of a forthcoming Marcus Mumford solo album began to leak out this past week, which countless radio stations and websites around the world immediately leaped to assume meant that old gang of his was breaking up. Mumford made it official on two counts — yes, his solo record is coming up, and no, Mumford and Sons are not splitting — and offered a first taste of the new project as he sat in with Brandi Carlile Friday at L.A.’s Greek Theater.During the encore portion of the first of Carlile’s two sold-out gigs at the Greek, the two sang a collaborative new ballad (the title of which was not announced) that Mumford said would be the closing track of his solo album, publicly confirming a heretofore secret project that has become widely speculated upon over the last week.