Make WorkingCopyBlueprint responsible for performing merges

Summary:
Ref T182. Currently, the "RepositoryLand" operation is responsible for performing merges when landing a revision.

However, we'd like to be able to perform these merges in a larger set of cases in the future. For example:

• After Releeph is revamped, when someone says "I want to merge bug fix X into stable branch Y", it would probably be nice to make that a Buildable and let tests run against it without requring that it actually be pushed anywhere.
• Same deal if we want a merge-from-Diffusion or cherry-pick-from-Diffusion operation.
• Similar deal if we want a "random web UI edits from Diffusion".

Move the merging part into WorkingCopy so more applications can share/use it in the future.

A big chunk of this is me making stuff up for now (the ol' undocumented dictionary full of arbitrary magic keys), but I anticipate formalizing it as we move along.

Test Plan: Pushed rGITTEST0d58eef3ce0fa5a10732d2efefc56aec126bc219 up from my local install via "Land Revision".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14337