Use hgsprintf() when managing bookmarks in arc patch in mercurial

Summary: Fixes T4596. I couldn't immediately reproduce this, but the `hgsprintf()` version is clearly more correct.

Test Plan: Used `arc patch --trace` to examine hg commands.

Reviewers: btrahan, durham

Reviewed By: durham

Subscribers: aran, epriestley

Maniphest Tasks: T4596

Differential Revision: https://secure.phabricator.com/D8512