svn copy fails with Subversion 1.5+ client if user does not have access to the Subversion root

Strange but true: if you reconfigure Studio to not use the 'users', 'developers' or 'administrators' groups, and you use a Subversion 1.5.x client to perform a 'copy' operation with a user not in any of these groups, eg:

svn cp https://example.jira.com/svn/TEST/trunk https://example.jira.com/svn/TEST/branches/1.0

The operation will fail, repeatedly prompting for your username and password:

Authentication realm: <https://example.jira.com:443> Subversion Repository
Password for 'jturner':
Authentication realm: <https://example.jira.com:443> Subversion Repository
Username: jturner
Password for 'jturner':
Authentication realm: <https://example.jira.com:443> Subversion Repository
Username: svn: PROPFIND of '/svntest': authorization failed (https://example.jira.com)

Other operations such as checkouts and commits work fine.