clang-format: Give clang-format-diff.py a -v option.

With it, it prints the file being formatted. Apparently people are
formatting thousands of files and some progress indication is helpful.

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@221990 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
diff --git a/clang-format-diff.py b/clang-format-diff.py
index b896ffc..247dace 100755
--- a/clang-format-diff.py
+++ b/clang-format-diff.py
@@ -52,6 +52,8 @@
                       r'|protodevel)',
                       help='custom pattern selecting file paths to reformat '
                       '(case insensitive, overridden by -regex)')
+  parser.add_argument('-v', '--verbose', action='store_true',
+                      help='be more verbose, ineffective without -i')
   parser.add_argument(
       '-style',
       help=
@@ -89,6 +91,8 @@
 
   # Reformat files containing changes in place.
   for filename, lines in lines_by_file.iteritems():
+    if args.i and args.verbose:
+      print 'Formatting', filename
     command = [binary, filename]
     if args.i:
       command.append('-i')