Make merge driver complain about older gits

clang_format_merge_driver.py

 #!/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]
+
+  if file_name_in_tree == '%P':
+    print >>sys.stderr
+    print >>sys.stderr, 'ERROR: clang-format merge driver needs git 2.5+'
+    if sys.platform == 'darwin':
+      print >>sys.stderr, 'Upgrade to Xcode 7.2+'
+    print >>sys.stderr
+    return 1
+
   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())