If the stars align, make "Land Revision" kind of work

Summary:
Ref T182. If 35 other things are configured completely correctly, make it remotely possible that this button may do something approximating the thing that the user wanted.

This primarily fleshes out the idea that "operations" (like landing, merging or cherry-picking) can have some beahavior, and when we run an operation we do whatever that behavior is instead of just running git show.

Broadly, this isn't too terrible because Drydock seems like it actually works properly for the most part (???!?!).

Test Plan: {F876431}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14270