Add a failing test of zero context.

src/rustfmt_diff.rs

@@ -169,7 +169,7 @@ mod test {
     use super::DiffLine::*;
 
     #[test]
-    fn simple_diff() {
+    fn diff_simple() {
         let src = "one\ntwo\nthree\nfour\nfive\n";
         let dest= "one\ntwo\ntrois\nfour\nfive\n";
         let diff = make_diff(src, dest, 1);
@@ -181,4 +181,16 @@ mod test {
                                              Context("four".into()),
                                          ] }]);
     }
+
+    #[test]
+    fn diff_zerocontext() {
+        let src = "one\ntwo\nthree\nfour\nfive\n";
+        let dest= "one\ntwo\ntrois\nfour\nfive\n";
+        let diff = make_diff(src, dest, 0);
+        assert_eq!(diff, vec![Mismatch { line_number: 3,
+                                         lines: vec![
+                                             Resulting("three".into()),
+                                             Expected("trois".into()),
+                                         ] }]);
+    }
 }