When tab-completing "arc" commands, suggest paths if the argument is empty and a path wildcard argument exists

Summary:
Currently, if you type "arc upload <tab>", we do not autocomplete the working directory. We should, but the "current argument" is the empty string and that's technically a prefix of every flag, so we suggest that you might want a flag instead.

You probably don't. Suggest paths in this case.

Test Plan:

• Ran "arc upload <tab>", saw path completions.
• Ran "arc land <tab>" (this workflow does NOT take paths as arguments), saw flag completions.

Differential Revision: https://secure.phabricator.com/D21385