nox-review: Fix --keep-going

[dezgeg]

Currently e.g. nox-review --keep-going wip --against HEAD^ fails
because extra-args is being set to [["--keep-going"]], that is a list
of list containing a string. This crashes nox later on:

Building in /tmp/nox-review-ig7cpfkp: clisp-tip gnu-smalltalk asymptote clisp maude ne libsigsegv urbit texlive.combined.scheme-full texlive.combined.scheme-medium tamarin-prover common-updater-scripts
Traceback (most recent call last):
  File "/nix/store/82f7gbp1byi5qp78rjrapzl7z97whjkp-nox-0.0.6/bin/.nox-review-wrapped", line 12, in <module>
    sys.exit(cli())
  File "/nix/store/n1zl5i5wclj4rakca9bpr3gzbzw7vaf1-python3.6-click-6.7/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/nix/store/n1zl5i5wclj4rakca9bpr3gzbzw7vaf1-python3.6-click-6.7/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/nix/store/n1zl5i5wclj4rakca9bpr3gzbzw7vaf1-python3.6-click-6.7/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/nix/store/n1zl5i5wclj4rakca9bpr3gzbzw7vaf1-python3.6-click-6.7/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/nix/store/n1zl5i5wclj4rakca9bpr3gzbzw7vaf1-python3.6-click-6.7/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/nix/store/n1zl5i5wclj4rakca9bpr3gzbzw7vaf1-python3.6-click-6.7/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/nix/store/82f7gbp1byi5qp78rjrapzl7z97whjkp-nox-0.0.6/lib/python3.6/site-packages/nox/review.py", line 71, in _
    f(*args, **kwargs)
  File "/nix/store/82f7gbp1byi5qp78rjrapzl7z97whjkp-nox-0.0.6/lib/python3.6/site-packages/nox/review.py", line 109, in wip
    build_in_path(ctx.obj['extra-args'], attrs, '.', dry_run=ctx.obj['dry_run'])
  File "/nix/store/82f7gbp1byi5qp78rjrapzl7z97whjkp-nox-0.0.6/lib/python3.6/site-packages/nox/review.py", line 38, in build_in_path
    click.echo('Invoking {}'.format(' '.join(command)))
TypeError: sequence item 1: expected str instance, list found

Remove the extra list to fix the crash.

[das-g]

It looks like 0d43970#diff-017d389e6f983a6d2eeee21343eb3c63R73 (and thereby master) already includes the same fix by now.