fix: unshallow clone for plugin checkout

[AntPeixe]

This will unshallow the clone otherwise no checkout can be done for commits older than head.

fixes #151

[mamaraddio]

Just a little tip.
Edit lines 43 and 44 and change it with

[[ -n "$git_ref" ]] && {
  git -C "$plugin_dir" pull --unshallow &> /dev/null
  git -C "$plugin_dir" checkout "$git_ref" &> /dev/null || { echo "❌ Failed to checkout $git_ref"; return 13 }
}

This is way more cleaner to read and the double failure message is useless

[AntPeixe]

Thanks for the tip, it's done

[mamaraddio]

👍