Merge branch 'jc/svn-auth-providers-unusable-at-1.6.12'

Regression fix for people with libsvn between 1.6.12 and 1.6.15, on which we tried to use the non-working platform auth providers. * jc/svn-auth-providers-unusable-at-1.6.12: git-svn: platform auth providers are working only on 1.6.15 or newer