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