Migrate notes.line_code format.

Since version 4.1, notes.line_code means:
  {SHA1 hash of file path}_{old line}_{new line}
but the older version means:
  {file index of the commit}_{old line}_{new line}

This rake task migrate the above differences.
Merge pull request #3911 from babatakao/migrate_note_linecode

Migrate notes.line_code format.