Improve diff op grouping for full matches

This changes the behavior of the diff op grouping function to return
an empty result if the diff is a full match.  This also adds an example
to demonstrate unified diff rendering.

examples/udiff.rs

@@ -0,0 +1,22 @@
+use std::fs::read_to_string;
+use std::process::exit;
+
+use similar::text::TextDiff;
+
+fn main() {
+    let args: Vec<_> = std::env::args_os().collect();
+    if args.len() != 3 {
+        eprintln!("usage: udiff [old] [new]");
+        exit(1);
+    }
+
+    let old = read_to_string(&args[1]).unwrap();
+    let new = read_to_string(&args[2]).unwrap();
+    print!(
+        "{}",
+        TextDiff::from_lines(&old, &new).unified_diff().header(
+            &args[1].as_os_str().to_string_lossy(),
+            &args[2].as_os_str().to_string_lossy()
+        )
+    );
+}