Fixing a bug when deleting conflict resolutions

Dependencies

• [2] CCLLB7OI Upgrading to Sanakirja 0.15 + version bump
• [3] QNJBR73K don't return Result for infallible functions
• [4] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).
• [5] UBCBQ5FG Removing pijul/src/commands/checkout.rs (unused file), as well as litorg comments

Change contents

• replacement in libpijul/src/diff/delete.rs at line 22
  [3.408]→[3.806025:806059](∅→∅),[2.116493]→[3.806025:806059](∅→∅),[3.806025]→[3.806025:806059](∅→∅)
  − debug!("delete {:?}", r);
  [2.116493]

  [2.116494]
  + debug!("delete {:?}: {:?}", r, d[r]);
• replacement in libpijul/src/diff/delete.rs at line 101
  [3.808811]→[3.808811:808868](∅→∅)
  − let first_vertex = diff.last_vertex_containing(pos);
  [3.808811]

  [3.808927]
  + let first_vertex = diff.first_vertex_containing(pos).max(1);

  + debug!("first_vertex = {:?}, vertex = {:?}", first_vertex, diff.pos_a[first_vertex].vertex);