Implemented ScoreManager._find_up_to_date_versioned_manager().

Used in tests.

Method finds an up-to-date ScorePackageManager or MaterialManager or whatever
sitting in either a Git or Svn repository.

Once the manager is found, items can be put into the package, inspected and
taken back out of the package, all during some filesystem-oriented test.

Currently being used to write add_to_repository() tests for manager classes.