rbt land is a command that can have some real impact on a source tree,

so it's important to let people feel comfortable knowing what it's going

to do.
This change adds a --dry-run argument that simulates the entire process

of landing a change, merging the branch, and pushing. It won't actually

perform those operations, but will act as if it did. This means that

some validation steps performed by the SCM tool may not occur, but will

still give a good sense of what's going to happen.
It also adds some output indicating what's going to happen at each

important stage, so that if something goes wrong, the user has a good

idea as to where and with what branches.

Tested with remote changes and local branches, with both squashing and merging.
Verified that the tree was untouched, but that all the expected operations were

being "performed" (as per the output).