Fix git cl format misunderstanding b/ diff prefixes

R=digit@chromium.org, maruel@chromium.org
BUG=244602

Review URL: https://chromiumcodereview.appspot.com/16007008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@202879 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -1964,7 +1964,7 @@ def CMDformat(parser, args):
     parser.error('Unrecognized args: %s' % ' '.join(args))
 
   # Generate diff for the current branch's changes.
-  diff_cmd = ['diff', '--no-ext-diff']
+  diff_cmd = ['diff', '--no-ext-diff', '--no-prefix']
   if opts.full:
     # Only list the names of modified files.
     diff_cmd.append('--name-only')