Add flag to specify path to google-java-format binary in google-java-format-diff.py

Fixes #44

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=218737500

Author: ngzhian

scripts/google-java-format-diff.py

@@ -31,8 +31,6 @@
 import sys
 from distutils.spawn import find_executable
 
-binary = find_executable('google-java-format') or '/usr/bin/google-java-format'
-
 def main():
   parser = argparse.ArgumentParser(description=
                                    'Reformat changed lines in diff. Without -i '
@@ -55,6 +53,7 @@ def main():
                       help='use AOSP style instead of Google Style (4-space indentation)')
   parser.add_argument('--skip-sorting-imports', action='store_true',
                       help='do not fix the import order')
+  parser.add_argument('-b', '--binary', help='path to google-java-format binary')
   args = parser.parse_args()
 
   # Extract changed lines for each file.
@@ -87,6 +86,11 @@ def main():
       lines_by_file.setdefault(filename, []).extend(
           ['-lines', str(start_line) + ':' + str(end_line)])
 
+  if args.binary:
+    binary = args.binary
+  else:
+    binary = find_executable('google-java-format') or '/usr/bin/google-java-format'
+
   # Reformat files containing changes in place.
   for filename, lines in lines_by_file.iteritems():
     if args.i and args.verbose: