rbt land command for landing changes in upstream

See https://reviewboard.hackpad.com/rbt-land-YrgGuhTuseU

for detailed description of the command.
What works:

Merging/rebasing a branch with/on destination branch.
Adding a commit message based on the review request.
Pushing to upstream
A wrapper around rbt patch in order to allow landing

others' changes.

Successfully merged and rebased a feature branch to/on master branch.


Successfully patched a review request from another user and landed it

on upstream.


Tested with:

empty repo
unclean working directory
unapproved review request
same target and destination branches
incorrect target/destination branch
incorrect upstream repository



and got appropriate error messages, as expected.