Implement -u option with no effect

If "svn diff --diff-cmd" is called without -x option, svn executes
external diff command with -u option. So this allows to use
difft as external diff for Apache Subversion, without extra options.

src/options.rs

@@ -140,6 +140,13 @@ fn app() -> clap::Command<'static> {
                 .validator(|s| s.parse::<u32>())
                 .required(false),
         )
+        .arg(
+            // Dummy 'unified' flag of POSIX/GNU diff utility for svn support:
+            // This takes no effect.
+            Arg::new("nop_unified")
+                .short('u')
+                .hide(true)
+        )
         .arg(
             Arg::new("label").short('L')
                 .long("label")