def modified_files
      staged = squash?
      refs = 'HEAD^ HEAD' if merge_commit?
      @modified_files ||= Overcommit::GitRepo.modified_files(staged: staged, refs: refs)
    end
def modified_files
      staged = squash?
      refs = 'HEAD^ HEAD' if merge_commit?
      @modified_files ||= Overcommit::GitRepo.modified_files(staged: staged, refs: refs)
    end
Get a list of files that were added, copied, or modified in the merge
 commit. Renames and deletions are ignored, since there should be nothing
 to check.