Fix tail -<number> does not work

[Smicry]

echo "a\nb" | ./target/debug/tail -1

b

[sylvestre]

Looks great. Could you please also add high level tests in:
https:/uutils/coreutils/blob/master/tests/by-util/test_tail.rs

And make sure that the coverage is great!

[sylvestre]

clippy is also complaining with:

error: field assignment outside of initializer for an instance created with Default::default()
   --> src/uu/tail/src/tail.rs:104:9
    |
104 |         settings.follow = matches.is_present(options::FOLLOW);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::field-reassign-with-default` implied by `-D warnings`
note: consider initializing the variable with `Settings { follow: matches.is_present(options::FOLLOW), ..Default::default() }` and removing relevant reassignments
   --> src/uu/tail/src/tail.rs:102:9
    |
102 |         let mut settings: Settings = Default::default();
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#field_reassign_with_default

error: this expression borrows a reference (`&Settings`) that is immediately dereferenced by the compiler
   --> src/uu/tail/src/tail.rs:200:41
    |
200 |             unbounded_tail(&mut reader, &settings);
    |                                         ^^^^^^^^^ help: change this to: `settings`
    |
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: this expression borrows a reference (`&Settings`) that is immediately dereferenced by the compiler
   --> src/uu/tail/src/tail.rs:232:41
    |
232 |                 bounded_tail(&mut file, &settings);
    |                                         ^^^^^^^^^ help: change this to: `settings`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: this expression borrows a reference (`&Settings`) that is immediately dereferenced by the compiler
   --> src/uu/tail/src/tail.rs:239:45
    |
239 |                 unbounded_tail(&mut reader, &settings);
    |                                             ^^^^^^^^^ help: change this to: `settings`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: this expression borrows a reference (`&Settings`) that is immediately dereferenced by the compiler
   --> src/uu/tail/src/tail.rs:248:34
    |
248 |         follow(&mut readers[..], &settings);
    |                                  ^^^^^^^^^ help: change this to: `settings`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: could not compile `uu_tail` due to 5 previous errors

[Smicry]

Looks great. Could you please also add high level tests in: https:/uutils/coreutils/blob/master/tests/by-util/test_tail.rs

And make sure that the coverage is great!

ok ~

[Smicry]

fixed #2507