Use Option's config_key for customizable dest branches in rbt land.

Review Request #6717 — Created Jan. 1, 2015 and submitted — Latest diff uploaded

Information

RBTools
master
ef12cf3...

Reviewers

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.