Rename instance: only show new name when different

It makes not sense to show messages like:
Fri May  6 02:04:01 2011  - INFO: Resolved given name 'instance18' to
'instance18'

So we'll skip the message if the resolved name is identical to the
requested one.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>