git_cl: add --parallel for yapf

see
https://github.com/google/yapf#usage

Bug: 1073708
Change-Id: I09a327e44bfcf5cf9a59c4e8d470a8886fc9f44b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2163086
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -4926,7 +4926,7 @@ def CMDformat(parser, args):
       if not yapf_style:
         yapf_style = 'pep8'
 
-      cmd = [yapf_tool, '--style', yapf_style, f]
+      cmd = [yapf_tool, '--parallel', '--style', yapf_style, f]
 
       has_formattable_lines = False
       if not opts.full: