tools: moveconfig: change class WorkDir to class ReferenceSource

tools: moveconfig: change class WorkDir to class ReferenceSource

The class WorkDir can be used in a very generic way, but currently
it is only used for containing a reference source directory.

This commit changes it for a more dedicated use.  The move_config
function can be more readable by enclosing the git-clone and git-
checkout in the class constructor.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>