Let git cl format be runnable outside of the root

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index bad3e657979cdb7ed6abe3f85c7412fbc36a5ffd..c401f0e762f20e421906e16a2fa4f2ac67a02783 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2248,6 +2248,12 @@ def CMDformat(parser, args):
   if args:
     parser.error('Unrecognized args: %s' % ' '.join(args))
 
+  # git diff generates paths against the root of the repository.  Change
+  # to that directory so clang-format can find files even within subdirs.
+  rel_base_path = RunGit(['rev-parse', '--show-cdup']).strip()
+  if rel_base_path:
+    os.chdir(rel_base_path)
+
   # Generate diff for the current branch's changes.
   diff_cmd = ['diff', '--no-ext-diff', '--no-prefix']
   if opts.full: