[Merge][#301969]Give proper error message for diff with non-existent dotted revno

[Aaron Bentley]

Aaron Bentley has voted resubmit.
Status is now: Resubmit
Comment:
It doesn't make sense to use NULL_REVISION when the revno is not 
supplied.  That is a specific revision-- it doesn't mean "unspecified".

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C418c22640811301225w58291dd1v9a691d390fe6ded5%40mail.gmail.com%3E
Project: Bazaar