rbt land had some custom code for allowing the destination branch to be
specified in .reviewboardrc, but it was just re-implementing the logic
that already lives in Option. Now we use Option's native support for
this, simplifying rbt land.

Tested with just a command line argument.
Tested with just a config setting in .reviewboardrc.
Tested without any defaults.
For each, I altered the code to print the value and compared to my expected value.