Fix mailbox_out/mailbox_in not round-tripping cleanly

RepoSurgeon.report_select(), invoked by mailbox_out, unconditionally appends a newline immediately following each commit comment. Unfortunately, this prevents mailbox_out/mailbox_in from round-tripping cleanly since a well-formed (newline-terminated) commit comment receives an extra (unneeded and unwanted) newline upon mailbox_out, which mailbox_in preserves on input. Thus, each commit comment grows longer (by a newline) on each round trip.