R1:5a35dd233b19

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…