Use new "%R" escape for csprintf() to produce slightly nicer clone/checkout commands

Summary: Fixes T4175. In cases where the arguments have only always-safe characters, we can produce a more human-readable URI.

Test Plan: Looked at some repositories.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4175

Differential Revision: https://secure.phabricator.com/D8100