Allow "arc" to accept any prefix of a command as that command

Summary:
Ref T13546. Practically, this allows "arc branch" to run "arc branches".

(This risks overcorrection to some degree, but command correction only occurs if stdout is a TTY so the risk seems limited.)

Test Plan: Ran "arc branch", got "arc branches" as a correction.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21338