Fix git cl format when it finds no clang-formattable files.

Currently, if git cl format does not find any clang-formattable files,
either in the path provided on the command line or in the CL as a whole,
then it applies clang-format to all changes in the CL. This CL fixes
this by changing git cl format to not clang-format when it does not find
any changed files that are clang-formattable.

Review URL: https://codereview.chromium.org/1734863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298971 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -3584,8 +3584,8 @@ def CMDformat(parser, args):
   # formatted. This is used to block during the presubmit.
   return_value = 0
 
-  if opts.full:
   if clang_diff_files:
+    if opts.full:
       cmd = [clang_format_tool]
       if not opts.dry_run and not opts.diff:
         cmd.append('-i')