feat(checkpkg): enhance diff command for comparing file lists

Replace `sdiff` with `diff` (also from `diffutils`) with the following
parameters:

- `--side-by-side` for the `sdiff`-like output
- `--suppress-common-lines` for the `sdiff -s` behavior
- `--width="$COLUMNS"` to use the full terminal width (long lines are
  still truncated but it is definitely better than the default width of
  130 chars)
- `--color=auto` just because 😉

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