Allow "+" to remain unescaped by csprintf('%R', ...)

Summary:
Ref T10923. Currently, when we %R ("readable") escape SVN SSH URIs (svn+ssh://...), they get quoted because they contain a +.

However, they don't need to be quoted, since + is not a special character in the shell.

Test Plan:

• Added and ran test cases.
• Viewed SVN in Diffusion, saw unquoted checkout URI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15882