pijul diff: println should really be writeln everywhere

Dependencies

• [2] I52XSRUH Massive cleanup, and simplification
• [3] YDKNUL6B Add `diff --short` that lists changes without showing them
• [4] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• replacement in pijul/src/commands/diff.rs at line 160
  [2.5591]→[2.5591:5627](∅→∅)
  − println!("{}", ch);
  [2.5591]

  [2.5627]
  + writeln!("{}", ch)?;