--- a/tools/mach_commands.py
+++ b/tools/mach_commands.py
@@ -198,41 +198,17 @@ class FormatProvider(MachCommandBase):
             clang_format_diff = self.locate_or_fetch(fmt_diff, python_script=True)
             if not clang_format_diff:
                 return 1
 
         except urllib2.HTTPError as e:
             print("HTTP error {0}: {1}".format(e.code, e.reason))
             return 1
 
-        from subprocess import Popen, PIPE
-
-        if os.path.exists(".hg"):
-            diff_process = Popen(["hg", "diff", "-U0", "-r", ".^",
-                                  "--include", "glob:**.c", "--include", "glob:**.cpp",
-                                  "--include", "glob:**.h",
-                                  "--exclude", "listfile:.clang-format-ignore"], stdout=PIPE)
-        else:
-            git_process = Popen(["git", "diff", "--no-color", "-U0", "HEAD^"], stdout=PIPE)
-            try:
-                diff_process = Popen(["filterdiff", "--include=*.h", "--include=*.cpp",
-                                      "--exclude-from-file=.clang-format-ignore"],
-                                     stdin=git_process.stdout, stdout=PIPE)
-            except OSError as e:
-                if e.errno == errno.ENOENT:
-                    print("Can't find filterdiff. Please install patchutils.")
-                else:
-                    print("OSError {0}: {1}".format(e.code, e.reason))
-                return 1
-
-        args = [sys.executable, clang_format_diff, "-p1"]
-        if not show:
-            args.append("-i")
-        cf_process = Popen(args, stdin=diff_process.stdout)
-        return cf_process.communicate()[0]
+        return self.run_clang_format_diff(clang_format_diff, show)
 
     def locate_or_fetch(self, root, python_script=False):
         import urllib2
         import hashlib
         bin_sha = {
             "Windows": "0cbfc306df48f01bfe804e5e89cef73b3abe8f884fb7a5208f8895897f19ec45c13760787298192bd37de057d0ded091640c7d504438e06ec880f071a38db89c",  # noqa: E501
             "Linux": "e6da4f6df074bfb15caefcf7767eb5670c02bb4768ba86ae4ab6b35235b53db012900a4f9e9a950ee140158a19532a71f21b986f511826bebc16f2ef83984e57",  # noqa: E501
             "Darwin": "18000940a11e5ab0c1fe950d4360292216c8e963dd708679c4c5fb8cc845f5919cef3f58a7e092555b8ea6b8d8a809d66153ea6d1e7c226a2c4f2b0b7ad1b2f3",  # noqa: E501
@@ -265,16 +241,43 @@ class FormatProvider(MachCommandBase):
                 return 1
             with open(temp, "wb") as fh:
                 fh.write(data)
                 fh.close()
             os.chmod(temp, os.stat(temp).st_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH)
             os.rename(temp, target)
         return target
 
+    def run_clang_format_diff(self, clang_format_diff, show):
+        from subprocess import Popen, PIPE
+
+        if os.path.exists(".hg"):
+            diff_process = Popen(["hg", "diff", "-U0", "-r", ".^",
+                                  "--include", "glob:**.c", "--include", "glob:**.cpp",
+                                  "--include", "glob:**.h",
+                                  "--exclude", "listfile:.clang-format-ignore"], stdout=PIPE)
+        else:
+            git_process = Popen(["git", "diff", "--no-color", "-U0", "HEAD^"], stdout=PIPE)
+            try:
+                diff_process = Popen(["filterdiff", "--include=*.h", "--include=*.cpp",
+                                      "--exclude-from-file=.clang-format-ignore"],
+                                     stdin=git_process.stdout, stdout=PIPE)
+            except OSError as e:
+                if e.errno == errno.ENOENT:
+                    print("Can't find filterdiff. Please install patchutils.")
+                else:
+                    print("OSError {0}: {1}".format(e.code, e.reason))
+                return 1
+
+        args = [sys.executable, clang_format_diff, "-p1"]
+        if not show:
+            args.append("-i")
+        cf_process = Popen(args, stdin=diff_process.stdout)
+        return cf_process.communicate()[0]
+
 
 def mozregression_import():
     # Lazy loading of mozregression.
     # Note that only the mach_interface module should be used from this file.
     try:
         import mozregression.mach_interface
     except ImportError:
         return None