Improve "next steps" text in Differential

Summary:

• Even for immutable-history Git workflows, we suggest "arc amend". Instead,

suggest "arc amend" or "arc merge" (ideally we'd know which, but we can't
currently get that information).

• We suggest "arc amend --revision X", but this is less safe and less simple

than "arc amend", especially after D1480.

• For Mercurial, suggest "arc merge".

Test Plan: Looked at some "Accepted" revisions.

Reviewers: btrahan, jungejason

CC: aran, epriestley

Maniphest Tasks: T662

Differential Revision: https://secure.phabricator.com/D1481