diff --git a/contrib/check-code.py b/contrib/check-code.py
--- a/contrib/check-code.py
+++ b/contrib/check-code.py
@@ -249,7 +249,8 @@
     (r"[^_]_\([ \t\n]*(?:'[^']+'[ \t\n+]*)+%", "don't use % inside _()"),
     (r'(\w|\)),\w', "missing whitespace after ,"),
     (r'(\w|\))[+/*\-<>]\w', "missing whitespace in expression"),
-    (r'^\s+(\w|\.)+=\w[^,()\n]*$', "missing whitespace in assignment"),
+#   false positive on multi-line keyword arguments
+#    (r'^\s+(\w|\.)+=\w[^,()\n]*$', "missing whitespace in assignment"),
     (r'\w\s=\s\s+\w', "gratuitous whitespace after ="),
     ((
         # a line ending with a colon, potentially with trailing comments
diff --git a/mercurial/thirdparty/xdiff.c b/mercurial/thirdparty/xdiff.c
new file mode 100644
--- /dev/null
+++ b/mercurial/thirdparty/xdiff.c
@@ -0,0 +1,96 @@
+/*
+ xdiff.c: simple Python wrapper for xdiff library
+
+ Copyright (c) 2018 Facebook, Inc.
+
+ This software may be used and distributed according to the terms of the
+ GNU General Public License version 2 or any later version.
+*/
+
+#include "xdiff.h"
+#include "Python.h"
+
+static int hunk_consumer(long a1, long a2, long b1, long b2, void *priv)
+{
+	PyObject *rl = (PyObject *)priv;
+	PyObject *m = Py_BuildValue("llll", a1, a2, b1, b2);
+	if (!m)
+		return -1;
+	if (PyList_Append(rl, m) != 0) {
+		Py_DECREF(m);
+		return -1;
+	}
+	return 0;
+}
+
+static PyObject *blocks(PyObject *self, PyObject *args)
+{
+	char *sa, *sb;
+	int na, nb;
+
+	if (!PyArg_ParseTuple(args, "s#s#", &sa, &na, &sb, &nb))
+		return NULL;
+
+	mmfile_t a = {sa, na}, b = {sb, nb};
+
+	PyObject *rl = PyList_New(0);
+	if (!rl)
+		return PyErr_NoMemory();
+
+	xpparam_t xpp = {
+	    0,    /* flags */
+	    NULL, /* anchors */
+	    0,    /* anchors_nr */
+	};
+	xdemitconf_t xecfg = {
+	    0,                  /* ctxlen */
+	    0,                  /* interhunkctxlen */
+	    XDL_EMIT_BDIFFHUNK, /* flags */
+	    NULL,               /* find_func */
+	    NULL,               /* find_func_priv */
+	    hunk_consumer,      /* hunk_consume_func */
+	};
+	xdemitcb_t ecb = {
+	    rl,   /* priv */
+	    NULL, /* outf */
+	};
+
+	if (xdl_diff(&a, &b, &xpp, &xecfg, &ecb) != 0) {
+		Py_DECREF(rl);
+		return PyErr_NoMemory();
+	}
+
+	return rl;
+}
+
+static char xdiff_doc[] = "xdiff wrapper";
+
+static PyMethodDef methods[] = {
+    {"blocks", blocks, METH_VARARGS,
+     "(a: str, b: str) -> List[(a1, a2, b1, b2)].\n"
+     "Yield matched blocks. (a1, a2, b1, b2) are line numbers.\n"},
+    {NULL, NULL},
+};
+
+static const int version = 1;
+
+#ifdef IS_PY3K
+static struct PyModuleDef bdiff_module = {
+    PyModuleDef_HEAD_INIT, "bdiff", xdiff_doc, -1, methods,
+};
+
+PyMODINIT_FUNC PyInit_xdiff(void)
+{
+	PyObject *m;
+	m = PyModule_Create(&xdiff_module);
+	PyModule_AddIntConstant(m, "version", version);
+	return m;
+}
+#else
+PyMODINIT_FUNC initxdiff(void)
+{
+	PyObject *m;
+	m = Py_InitModule3("xdiff", methods, xdiff_doc);
+	PyModule_AddIntConstant(m, "version", version);
+}
+#endif
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -220,7 +220,7 @@
     py2exe.Distribution # silence unused import warning
     py2exeloaded = True
     # import py2exe's patched Distribution class
-    from distutils.core import Distribution
+    from distutils.core import Distribution # noqa: F811
 except ImportError:
     py2exeloaded = False
 
@@ -874,6 +874,28 @@
               extra_compile_args=osutil_cflags,
               extra_link_args=osutil_ldflags,
               depends=common_depends),
+    Extension('mercurial.cext.xdiff',
+              sources=[
+                  'mercurial/thirdparty/xdiff/xdiffi.c',
+                  'mercurial/thirdparty/xdiff/xemit.c',
+                  'mercurial/thirdparty/xdiff/xmerge.c',
+                  'mercurial/thirdparty/xdiff/xprepare.c',
+                  'mercurial/thirdparty/xdiff/xutils.c',
+                  'mercurial/thirdparty/xdiff.c',
+              ],
+              include_dirs=common_include_dirs + [
+                  'mercurial/thirdparty/xdiff',
+              ],
+              depends=common_depends + [
+                  'mercurial/thirdparty/xdiff/xdiff.h',
+                  'mercurial/thirdparty/xdiff/xdiffi.h',
+                  'mercurial/thirdparty/xdiff/xemit.h',
+                  'mercurial/thirdparty/xdiff/xinclude.h',
+                  'mercurial/thirdparty/xdiff/xmacros.h',
+                  'mercurial/thirdparty/xdiff/xprepare.h',
+                  'mercurial/thirdparty/xdiff/xtypes.h',
+                  'mercurial/thirdparty/xdiff/xutils.h',
+              ]),
     Extension('hgext.fsmonitor.pywatchman.bser',
               ['hgext/fsmonitor/pywatchman/bser.c']),
     ]