Add a "%?" ("hint") conversion to "tsprintf()"

Summary: Ref T13546. Future "arc land" workflows use this element to provide "hint" or "next step" suggestions to make error resolution easier.

Test Plan: Ran future "arc land" workflows, saw tips like "use --revision to do such-and-such" or "add these files to .gitignore".

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21309