set proper sys.argv[0] for subcommand

[bnavigator]

Hi,

In openSUSE/libalternatives#11 we noticed that jupyter subcommand calls jupyter-subcommand with an argv[0]=subcommand instead of the proper executable name jupyter-subcommand.

Most subcommands do not care, because the first thing they do, is strip sys.argv[0] from the parsing:

jupyter_core/jupyter_core/application.py

Lines 227 to 230 in 1778a44

	if argv is None:
	argv = sys.argv[1:]
	if argv:
	subc = self._find_subcommand(argv[0])

From the Python Doc: os.execv*():

In either case, the arguments to the child process should start with the name of the command being run, but this is not enforced.

So nothing mandates argv[0] to be set to the name, but it is good practise and libalternatives relies on it.

You already do the same for subcommands of JupyterApps:

jupyter_core/jupyter_core/application.py

Lines 246 to 249 in 1778a44

	def start(self):
	"""Start the whole thing"""
	if self.subcommand:
	os.execv(self.subcommand, [self.subcommand] + self.argv[1:])

[jasongrout]

Thanks! Can we also add a test in https://github.com/jupyter/jupyter_core/blob/master/jupyter_core/tests/test_command.py to make sure we don't break this in the future?

[jasongrout]

@bnavigator - does the test I added look good to you?

[bnavigator]

LGTM