tail: fix off-by-one issue for +NUM args

Fix an off-by-one issue for `tail -c +NUM` and `tail -n +NUM` command
line options.

src/uu/tail/src/tail.rs

@@ -489,7 +489,10 @@ where
     E: fmt::Debug,
 {
     if beginning {
-        iter.skip(count as usize).map(|r| r.unwrap()).collect()
+        // GNU `tail` seems to index bytes and lines starting at 1, not
+        // at 0. It seems to treat `+0` and `+1` as the same thing.
+        let i = count.max(1) - 1;
+        iter.skip(i as usize).map(|r| r.unwrap()).collect()
     } else {
         RingBuffer::from_iter(iter.map(|r| r.unwrap()), count as usize).data
     }

tests/by-util/test_tail.rs

@@ -348,3 +348,47 @@ fn test_negative_indexing() {
 fn test_sleep_interval() {
     new_ucmd!().arg("-s").arg("10").arg(FOOBAR_TXT).succeeds();
 }
+
+
+/// Test for reading all but the first NUM bytes: `tail -c +3`.
+#[test]
+fn test_positive_bytes() {
+    new_ucmd!()
+        .args(&["-c", "+3"])
+        .pipe_in("abcde")
+        .succeeds()
+        .stdout_is("cde");
+}
+
+
+/// Test for reading all bytes, specified by `tail -c +0`.
+#[test]
+fn test_positive_zero_bytes() {
+    new_ucmd!()
+        .args(&["-c", "+0"])
+        .pipe_in("abcde")
+        .succeeds()
+        .stdout_is("abcde");
+}
+
+
+/// Test for reading all but the first NUM lines: `tail -n +3`.
+#[test]
+fn test_positive_lines() {
+    new_ucmd!()
+        .args(&["-n", "+3"])
+        .pipe_in("a\nb\nc\nd\ne\n")
+        .succeeds()
+        .stdout_is("c\nd\ne\n");
+}
+
+
+/// Test for reading all lines, specified by `tail -n +0`.
+#[test]
+fn test_positive_zero_lines() {
+    new_ucmd!()
+        .args(&["-n", "+0"])
+        .pipe_in("a\nb\nc\nd\ne\n")
+        .succeeds()
+        .stdout_is("a\nb\nc\nd\ne\n");
+}