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