Fixing a panic in `pijul change`

Dependencies

• [2] Q4SVMHAE Removing --channel from the changes command
• [3] JG3MWHEN commands/change: help messages
• [4] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• edit in pijul/src/commands/change.rs at line 24
  [3.189846]→[2.279:345](∅→∅),[2.345]→[3.189929:189992](∅→∅),[3.189929]→[3.189929:189992](∅→∅)
  − let channel_name = repo.config.get_current_channel(None);

  − let channel = txn.load_channel(channel_name).unwrap();
• edit in pijul/src/commands/change.rs at line 28
  [3.190123]→[3.190123:190229](∅→∅)
  − } else if let Some((_, (h, _))) = txn.reverse_log(&channel.borrow(), None).next() {

  − h
• replacement in pijul/src/commands/change.rs at line 29
  [3.190246]→[3.190246:190273](∅→∅)
  − return Ok(());
  [3.190246]

  [3.190273]
  + let channel_name = repo.config.get_current_channel(None);

  + let channel = if let Some(channel) = txn.load_channel(channel_name) {

  + channel

  + } else {

  + return Ok(());

  + };

  + let channel = channel.borrow();

  + if let Some((_, (h, _))) = txn.reverse_log(&channel, None).next() {

  + h

  + } else {

  + return Ok(());

  + }