devtool: modify: Make --no-extract work again

devtool: modify: Make --no-extract work again

This avoids the following error when using --no-extract, introduced in
commit 900129cbdf (devtool: add support for git submodules):

  Traceback (most recent call last):
    File ".../scripts/devtool", line 349, in <module>
      ret = main()
    File ".../scripts/devtool", line 336, in main
      ret = args.func(args, config, basepath, workspace)
    File ".../scripts/lib/devtool/standard.py", line 995, in modify
      for commit in commits[name]:
  KeyError: '.'

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>