Add a flag for clang-format that instructs the tool to strip 0 leading slashes from the file names.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 43c9d365bd93c3a0d1dabe1d2d6372fd8a8a297d..2f41bda957811150c2ff17da466289ac1ddd594f 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2181,7 +2181,7 @@ def CMDformat(parser, args):
                             'clang-format-diff.py')
     if not os.path.exists(cfd_path):
       DieWithError('Could not find clang-format-diff at %s.' % cfd_path)
-    cmd = [sys.executable, cfd_path, '-style', 'Chromium']
+    cmd = [sys.executable, cfd_path, '-p0', '-style', 'Chromium']
     RunCommand(cmd, stdin=diff_output, cwd=top_dir)
 
   return 0