justfile: add recipes for updating mirrors

Dependencies

• [2] 4D7RX5B6 final commit before workshop
• [*] HME56U35 start building slides in org-mode

Change contents

• edit in justfile at line 19
  [2.2627466]
  +

  + update-mirrors: update-hg update-pijul update-darcs

  +

  + [working-directory: 'mirrors/hg']

  + update-hg:

  + hg pull {{ justfile_directory() }}

  +

  + [working-directory: 'mirrors/pijul']

  + update-pijul:

  + pijul git {{ justfile_directory() }}

  +

  + [working-directory: 'mirrors']

  + update-darcs:

  + rm -rf darcs

  + (cd ..; git fast-export --all --progress 500) | darcs convert import darcs