Arc output the correct action being taken

Summary: Fixes T3929. If arc is merging it will say merging * into * vs always saying rebasing * onto *

Test Plan: Make arc do a rebase, then a merge

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3929

Differential Revision: https://secure.phabricator.com/D8085