Introduce git merge driver for the blink reformatting

Based on CL from primiano@: https://codereview.chromium.org/2348793003/

This is a simple tool that can help automatically resolve conflicts
from clang-format reformatting patches.

BUG=574611

Review-Url: https://codereview.chromium.org/2356933002

clang_format_merge_driver

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+base_dir=$(dirname "$0")
+
+PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/clang_format_merge_driver.py" "$@"

clang_format_merge_driver.bat

@@ -0,0 +1,11 @@
+@echo off
+:: Copyright 2016 The Chromium Authors. All rights reserved.
+:: Use of this source code is governed by a BSD-style license that can be
+:: found in the LICENSE file.
+setlocal
+
+:: This is required with cygwin only.
+PATH=%~dp0;%PATH%
+
+:: Defer control.
+%~dp0python "%~dp0\clang_format_merge_driver.py" %*

clang_format_merge_driver.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""clang-format 3-way merge driver.
+
+This is a custom merge driver for git that helps automatically resolves
+conflicts caused by clang-format changes. The conflict resolution
+strategy is extremely simple: it simply clang-formats the current,
+ancestor branch's, and other branch's version of the file and delegates
+the remaining work to git merge-file.
+
+See https://git-scm.com/docs/gitattributes ("Defining a custom merge
+driver") for more details.
+"""
+
+import subprocess
+import sys
+
+import clang_format
+
+
+def main():
+  if len(sys.argv) < 5:
+    print('usage: %s <base> <current> <others> <path in the tree>' %
+          sys.argv[0])
+    sys.exit(1)
+
+  base, current, others, file_name_in_tree = sys.argv[1:5]
+  print 'Running clang-format 3-way merge driver on ' + file_name_in_tree
+
+  try:
+    tool = clang_format.FindClangFormatToolInChromiumTree()
+    for fpath in base, current, others:
+      # Typically, clang-format is used with the -i option to rewrite files
+      # in-place. However, merge files live in the repo root, so --style=file
+      # will always pick up the root .clang-format.
+      #
+      # Instead, this tool uses --assume-filename so clang-format will pick up
+      # the appropriate .clang-format. Unfortunately, --assume-filename only
+      # works when the input is from stdin, so the file I/O portions are lifted
+      # up into the script as well.
+      with open(fpath, 'rb') as input_file:
+        output = subprocess.check_output(
+            [tool, '--assume-filename=%s' % file_name_in_tree, '--style=file'],
+            stdin=input_file)
+      with open(fpath, 'wb') as output_file:
+        output_file.write(output)
+  except clang_format.NotFoundError, e:
+    print e
+    print 'Failed to find clang-format. Falling-back on standard 3-way merge'
+
+  return subprocess.call(['git', 'merge-file', '-Lcurrent', '-Lbase', '-Lother',
+                          current, base, others])
+
+
+if __name__ == '__main__':
+  sys.exit(main())