Don't use --ff-only inside "Land Revision"

Summary:
Ref T182. I lifted this logic out of `arc`, but the context is a little different there, and this option is too strict in "Land Revision".

Specifically, it prevents `git` from merging unless the merge is //strictly// a fast-foward, even with `--squash`. That means revisions can't merge unless they're rebased on the current `master`, even if they have no conflicts.

(This whole process will probably need additional refinement, but the behavior without this flag is more reasonable overall than the behavior with it for now.)

Test Plan: Will land stuff in production~~

Reviewers: chad, Mnkras

Reviewed By: Mnkras

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14346