diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -7,6 +7,8 @@
 
 from __future__ import absolute_import
 
+import datetime
+import itertools
 import re
 import struct
 import zlib
@@ -525,3 +527,33 @@
 
 def replacediffheader(oldlen, newlen):
     return struct.pack(">lll", 0, oldlen, newlen)
+
+def prepare_mdiff(expected, output):
+    """Prepare the inputs for the mdiff.unidiff function"""
+    date1 = datetime.datetime.now().strftime("%a %b %d %y %H:%M:%S %Y %z")
+    date2 = date1
+    # join all the different elements into a single string
+    # to regenerate that file
+    exp = "".join(expected)
+    out = "".join(output)
+    opts = diffopts(noprefix=True)
+    return exp, date1, out, date2, opts
+
+def process_mdiff(uheaders, hunks):
+    """Process the output of mdiff into a list of lines,
+    to be used by getdiff"""
+    # the hunklines are in the hunks generator
+    # get the hunklines from the (hunkrange,hunklines) tuple
+    hunklines = [x[1] for x in hunks]
+    # extract and insert the headers at the beginnng of the hunklines list
+    headers = [uheaders[0].split("\t")[0]+"\n", uheaders[1].split("\t")[0]+"\n"]
+    hunklines.insert(0, headers)
+    difflines = itertools.chain.from_iterable(hunklines)
+    return difflines
+
+def new_diff(expected, output, ref, err):
+    exp, date1, out, date2, opts = prepare_mdiff(expected, output)
+    uheaders, hunks = unidiff(exp, date1, out, date2,
+                            ref, err, binary=False, opts=opts)
+    difflines = process_mdiff(uheaders, hunks)
+    return difflines
diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -47,9 +47,11 @@
 
 import argparse
 import collections
+import datetime
 import difflib
 import distutils.version as version
 import errno
+import itertools
 import json
 import multiprocessing
 import os
@@ -68,6 +70,18 @@
 import uuid
 import xml.dom.minidom as minidom
 
+_unified_diff = difflib.unified_diff
+if PYTHON3:
+    import functools
+    _unified_diff = functools.partial(difflib.diff_bytes, difflib.unified_diff)
+
+try:
+    from mercurial import mdiff, patch
+    _diff = mdiff.new_diff
+except Exception:
+    print("Falling back to unified_diff")
+    _diff = _unified_diff
+
 try:
     import Queue as queue
 except ImportError:
@@ -598,15 +612,10 @@
             except OSError:
                 pass
 
-_unified_diff = difflib.unified_diff
-if PYTHON3:
-    import functools
-    _unified_diff = functools.partial(difflib.diff_bytes, difflib.unified_diff)
-
 def getdiff(expected, output, ref, err):
     servefail = False
     lines = []
-    for line in _unified_diff(expected, output, ref, err):
+    for line in _diff(expected, output, ref, err):
         if line.startswith(b'+++') or line.startswith(b'---'):
             line = line.replace(b'\\', b'/')
             if line.endswith(b' \n'):