[Subversion] Fix spam of TooOld/Upgrading dialogs.

Due to SvnClient initializing a new Backend on every check, the value wasn't retained between Status queries.
This is only a temporary fix, the actual fix for this would be abstracting the backend into actual backend code and interop wrappers to simulate SvnSharp.
Merge pull request #661 from mono/fixSvnWCFormat

[Subversion] Fix spam of TooOld/Upgrading dialogs.