Changing --amend flag to respect -m flag

Dependencies

• [2] MF3WAHBI commands/record: amend current change if unspecified
• [3] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• replacement in pijul/src/commands/record.rs at line 81
  [2.355]→[3.103866:103921](∅→∅),[3.103866]→[3.103866:103921](∅→∅)
  − let header = repo.changes.get_header(&h)?;
  [2.355]

  [3.103921]
  + let header = if let Some(message) = self.message.clone() {

  + ChangeHeader {

  + message,

  + ..repo.changes.get_header(&h)?

  + }

  + } else {

  + repo.changes.get_header(&h)?

  + };

  +