add unified diff separator

 - fix issue mochajs#2295
 - the purpose of this feature is to make the unified diff
   format more readable.  Without it, different portions
   of the same actual/expected diff look contiguous even
   though they are actually separated by the default
   context of four lines.

lib/reporters/base.js

@@ -393,7 +393,7 @@ function inlineDiff (err, escape) {
  */
 function unifiedDiff (err, escape) {
   var indent = '      ';
-  function cleanUp (line) {
+  function cleanUp (line, idx) {
     if (escape) {
       line = escapeInvisibles(line);
     }
@@ -404,7 +404,7 @@ function unifiedDiff (err, escape) {
       return indent + colorLines('diff removed', line);
     }
     if (line.match(/@@/)) {
-      return null;
+      return '--';
     }
     if (line.match(/\\ No newline/)) {
       return null;
@@ -415,7 +415,7 @@ function unifiedDiff (err, escape) {
     return typeof line !== 'undefined' && line !== null;
   }
   var msg = diff.createPatch('string', err.actual, err.expected);
-  var lines = msg.split('\n').splice(4);
+  var lines = msg.split('\n').splice(5);
   return '\n      ' +
     colorLines('diff added', '+ expected') + ' ' +
     colorLines('diff removed', '- actual') +

test/reporters/base.spec.js

@@ -14,6 +14,14 @@ function makeTest (err) {
   };
 }
 
+function createElements (argObj) {
+  var res = [];
+  for (var i = argObj.from; i <= argObj.to; i += 1) {
+    res.push('element ' + i);
+  }
+  return res;
+}
+
 describe('Base reporter', function () {
   var stdout;
   var stdoutWrite;
@@ -160,6 +168,46 @@ describe('Base reporter', function () {
     });
   });
 
+  describe('unified diff reporter', function () {
+    it('should separate diff hunks by two dashes', function () {
+      var err = new Error('test');
+      var errOut;
+
+      err.actual = createElements({ from: 2, to: 11 });
+      err.expected = createElements({ from: 1, to: 10 });
+      err.showDiff = true;
+      var test = makeTest(err);
+
+      Base.inlineDiffs = false;
+      Base.list([test]);
+
+      errOut = stdout.join('\n');
+
+      var regexesToMatch = [
+        /\[/,
+        /\+ {2}"element 1"/,
+        /"element 2"/,
+        /"element 3"/,
+        /"element 4"/,
+        /"element 5"/,
+        /--/,
+        /"element 7"/,
+        /"element 8"/,
+        /"element 9"/,
+        /"element 10"/,
+        /- {2}"element 11"/,
+        /]/,
+        /test/,
+        /expected/,
+        /actual/
+      ];
+
+      regexesToMatch.forEach(function (aRegex) {
+        errOut.should.match(aRegex);
+      });
+    });
+  });
+
   it('should stringify objects', function () {
     var err = new Error('test');
     var errOut;