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.

src/algorithms/capture.rs

@@ -222,8 +222,9 @@ pub fn group_diff_ops(mut ops: Vec<DiffOp>, n: usize) -> Vec<Vec<DiffOp>> {
         pending_group.push(op);
     }
 
-    if !pending_group.is_empty() {
-        rv.push(pending_group);
+    match &pending_group[..] {
+        &[] | &[DiffOp::Equal { .. }] => {}
+        _ => rv.push(pending_group),
     }
 
     rv

src/text/snapshots/similar__text__udiff__unified_diff.snap

@@ -0,0 +1,25 @@
+---
+source: src/text/udiff.rs
+expression: "&diff.unified_diff().header(\"a.txt\", \"b.txt\").to_string()"
+---
+--- a.txt
++++ b.txt
+@@ -15,3 +19,3 @@
+ p
+ q
+ r
+-s
++S
+ t
+ u
+ v
+@@ -37,3 +41,3 @@
+ L
+ M
+ N
+-O
++o
+ P
+ Q
+ R
+

src/text/udiff.rs

@@ -178,8 +178,12 @@ impl<'diff, 'old, 'new, 'bufs> fmt::Display for UnifiedDiffHunk<'diff, 'old, 'ne
         } else {
             "\n"
         };
-        writeln!(f, "{}", self.header())?;
+        let mut wrote_header = false;
         for change in self.iter_changes() {
+            if !wrote_header {
+                writeln!(f, "{}", self.header())?;
+                wrote_header = true;
+            }
             write!(
                 f,
                 "{}{}{}",
@@ -238,3 +242,8 @@ fn test_unified_diff() {
     );
     insta::assert_snapshot!(&diff.unified_diff().header("a.txt", "b.txt").to_string());
 }
+#[test]
+fn test_empty_unified_diff() {
+    let diff = TextDiff::from_lines("abc", "abc");
+    assert_eq!(diff.unified_diff().header("a.txt", "b.txt").to_string(), "");
+}