fts: three levels of leaf optimization * lib/fts.c (enum leaf_optimization): New type with three values. (S_MAGIC_AFS): New macro. Sort them. (leaf_optimization): Rename from leaf_optimization_applies, and return enum leaf_optimization instead of bool. All uses changed. Add cases for unknown type and for AFS. (fts_build): Don’t rely on link counts if NO_LEAF_OPTIMIZATION.