Fix the concatenation of repository root and file when recording

Dependencies

• [2] ZHABNS3S Canonicalize all paths
• [3] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• replacement in libpijul/src/working_copy/filesystem.rs at line 55
  [3.209394]→[3.209394:209431](∅→∅),[3.209431]→[2.1532:1571](∅→∅)
  − debug!("get prefix {:?}", repo);

  − let prefix_ = repo.join(&prefix)?;
  [3.209394]

  [3.209577]
  + debug!("get prefix {:?} {:?}", repo, prefix);

  + let prefix_ = CanonicalPathBuf::canonicalize(&repo.as_path().join(&prefix))?;