oeqa/selftest: add test case to cover 'devtool modify -n' for a git recipe

oeqa/selftest: add test case to cover 'devtool modify -n' for a git recipe

Add a test case to ensure the following error does not happen again for
'devtool modify -n'.

Traceback (most recent call last):
  File "/buildarea2/chenqi/poky/scripts/devtool", line 349, in <module>
    ret = main()
  File "/buildarea2/chenqi/poky/scripts/devtool", line 336, in main
    ret = args.func(args, config, basepath, workspace)
  File "/buildarea2/chenqi/poky/scripts/lib/devtool/standard.py", line 924, in modify
    if not initial_revs["."]:
KeyError: '.'

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>