help: print byte slice defaults as strings

There are a couple of other places where []u8 is treated implicitly
like a string, which isn't strictly correct. Ultimately, some kind of
metasignal will be required to make this type truly unambiguous in
interpretation.

source/help.zig

@@ -471,7 +471,17 @@ pub fn opt_info(comptime command: anytype) CommandHelp {
                 // TODO: this is only acceptable for some types. It behaves poorly on
                 // enum-based choice types because it prints the whole type name rather
                 // than just the tag name. Roll our own eventually.
+                blk: {
+                    switch (@typeInfo(@TypeOf(def))) {
+                        .Pointer => |info| if (info.size == .Slice and info.child == u8) {
+                            writer.print("{s}", .{def}) catch @compileError("no");
+                            break :blk;
+                        },
+                        else => {},
+                    }
                     writer.print("{any}", .{def}) catch @compileError("whoah");
+                }
+
                 last_option.default = buf.buffer;
             }
         }