Add a new --vacuum-pristines option to 'svn cleanup'.
This option toggles the 'vacuum_pristines' API flag of svn_client_vacuum() and allows cleaning up the pristine store with a working copy lock held. It avoids a risk of working copy corruption due to concurrent access by multiple clients. Because 'svn cleanup' ignores and removes any locks in its default mode, using that mode to clean up pristines is subtly unsafe.
Adding this feature is cheap since the libsvn_client API already provides it. The command line client just didn't expose it yet.