Port "arc prompts" from wilds and fix a path issue in shell completion
Ref T13490. Bring "arc prompts" from "wilds" and hook it into the prompt in "arc shell-complete".
See D21069. Fix an issue where the shell hook tested for a path other than the path it writes to.
Test Plan: Ran "arc shell-complete" with no hook and got a prompt. Shell completed things. Ran "arc prompts shell-complete".
Maniphest Tasks: T13490
Differential Revision: https://secure.phabricator.com/D21085