Fix breaking the 'clean' subcommand caused replacing a single-letter …

…variable with the same value in two contexts where it was used differently. That's why you don't use "m" as a variable for hundreds of lines in an outer function, and re-use it in closures several times in the same function. Sheesh.

src/bootstrap/flags.rs

@@ -244,7 +244,7 @@ Arguments:
                 Subcommand::Doc { paths: paths }
             }
             "clean" => {
-                if matches.free.len() > 0 {
+                if paths.len() > 0 {
                     println!("\nclean takes no arguments\n");
                     usage(1, &opts, &subcommand_help, &extra_help);
                 }