Add "%R" to csprintf(), for printing more readable commands

Summary: Ref T4175. The clone commands we print have quoted arguments, but usually don't need them. Add "%R", which is like "%s" but only adds quotes if they're required. This produces slightly more readable commands in the few cases we're generating commands for users to copy/paste.

Test Plan: Added unit tests. Looked up shell metacharacters to double check my definition.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4175

Differential Revision: https://secure.phabricator.com/D8099