Can not pass target dir without also passing branch name

[yuvipanda]

Bug description

#185 adds autodetecting branch name on omission. But now we have two consecutive optional args on the commandline - branch and repo_dir (target clone path). So if we want to pass just the repo url and repo_dir, it's currently not possible.

Expected behaviour

Tell nbgitpuller to autodetect branch name but specify target directory when called from the commandline

Actual behaviour

If only two arguments are specified on the commandline, they are counted as repo url and branch, not repo url and repo_dir

How to reproduce

❯ gitpuller https://github.com/jupyterhub/jupyterhub target-dir                                           2 ↵
Traceback (most recent call last):
  File "/Users/yuvipanda/.local/share/virtualenvs/nbgitpuller/bin/gitpuller", line 33, in <module>
    sys.exit(load_entry_point('nbgitpuller', 'console_scripts', 'gitpuller')())
  File "/Users/yuvipanda/code/nbgitpuller/nbgitpuller/pull.py", line 310, in main
    for line in GitPuller(
  File "/Users/yuvipanda/code/nbgitpuller/nbgitpuller/pull.py", line 78, in __init__
    raise ValueError(f"Branch: {self.branch_name} -- not found in repo: {self.git_url}")
ValueError: Branch: target-dir -- not found in repo: https://github.com/jupyterhub/jupyterhub

[sean-morris]

I will make the command line arguments keyword instead of positional. The argparser can handle it. Should have looked that up! when i ran into the problem.

[yuvipanda]

I think this was merged in #185