Minor cleanup with default

Dependencies

• [2] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).
• [3] PSKXR4QE Do not load the entire change in memory in log
• [4] 6YMDOZIB Refactoring apply

Change contents

• replacement in libpijul/src/change.rs at line 111
  [2.833365]→[2.833365:833404](∅→∅)
  − #[serde(default = "default_none")]
  [2.833365]

  [2.833404]
  + #[serde(default)]
• replacement in libpijul/src/change.rs at line 113
  [2.833439]→[2.833439:833478](∅→∅)
  − #[serde(default = "default_none")]
  [2.833439]

  [2.833478]
  + #[serde(default)]
• edit in libpijul/src/change.rs at line 115
  [2.833509]→[2.833509:833559](∅→∅)
  − }

  −

  − fn default_none() -> Option<String> {

  − None