Setting the oldest modification time to EPOCH if no file has changed

Dependencies

• [2] DJYHARZ7 Skipping old files when recording
• [3] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).
• [4] 7FFFKQZU add 'Default' implementations

Change contents

• replacement in libpijul/src/record.rs at line 92
  [2.1283]→[2.1283:1344](∅→∅)
  − oldest_change: std::time::SystemTime::now(),
  [2.1283]

  [2.1344]
  + oldest_change: std::time::SystemTime::UNIX_EPOCH,
• replacement in libpijul/src/record.rs at line 463
  [2.1644]→[2.1644:1736](∅→∅)
  − self.rec.oldest_change = self.rec.oldest_change.min(last_modified);
  [2.1644]

  [2.1736]
  + if self.rec.oldest_change == std::time::SystemTime::UNIX_EPOCH {

  + self.rec.oldest_change = last_modified;

  + } else {

  + self.rec.oldest_change = self.rec.oldest_change.min(last_modified);

  + }