nl: fix calculation of line number lengths

[cakebaker]

This PR fixes the indentation of line numbers by adapting the calculation of the line number lengths to work with negative line numbers (introduced in #5078).

// before change
$ printf "a\nb\nc" | cargo run nl -i10 -v-19
   -19  a
   -9   b
   1    c

// after change
$ printf "a\nb\nc" | cargo run nl -i10 -v-19
   -19  a
    -9  b
     1  c

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail-2/inotify-dir-recreate

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail-2/inotify-dir-recreate