Function label_of_buffer 

pub fn label_of_buffer(store: &BufferStoreHandle, buffer: BufferId) -> String

MR.3b: the repository label a magit buffer’s own name carries, for a producer that has the buffer store but no services.

Empty when the buffer is not a magit buffer or carries no label — which composes correctly with the name producers, since an empty label is the outside-a-repository form.