aur-repo: support setting db extension with AUR_DBEXT

For example if custom.db is not a symlink (on file systems not
supporting them) but a regular file:

  $ AUR_DBEXT=db.tar.gz aur repo -d custom
  /path/to/repo/custom.db.tar.gz

  $ aur repo -d custom
  /path/to/repo/custom.db

To achieve this in aur-sync, extend --status-file to include the
repository path.