head: -c allocate memory as needed (#5704)

* head: -c allocate memory as needed

Improve the execution of tests/head/head-c.sh

* simplify the code

Co-authored-by: Daniel Hofstetter <daniel.hofstetter@42dh.com>

---------

Co-authored-by: Daniel Hofstetter <daniel.hofstetter@42dh.com>

src/uu/head/src/head.rs

@@ -252,43 +252,37 @@ fn read_but_last_n_bytes(input: &mut impl std::io::BufRead, n: usize) -> std::io
     let stdout = std::io::stdout();
     let mut stdout = stdout.lock();
 
-    let mut ring_buffer = vec![0u8; n];
+    let mut ring_buffer = Vec::new();
 
-    // first we fill the ring buffer
-    if let Err(e) = input.read_exact(&mut ring_buffer) {
-        if e.kind() == ErrorKind::UnexpectedEof {
-            return Ok(());
-        } else {
-            return Err(e);
-        }
-    }
     let mut buffer = [0u8; BUF_SIZE];
+    let mut total_read = 0;
+
     loop {
-        let read = loop {
-            match input.read(&mut buffer) {
-                Ok(n) => break n,
-                Err(e) => match e.kind() {
-                    ErrorKind::Interrupted => {}
-                    _ => return Err(e),
-                },
-            }
+        let read = match input.read(&mut buffer) {
+            Ok(0) => break,
+            Ok(read) => read,
+            Err(e) => match e.kind() {
+                ErrorKind::Interrupted => continue,
+                _ => return Err(e),
+            },
         };
-        if read == 0 {
-            return Ok(());
-        } else if read >= n {
-            stdout.write_all(&ring_buffer)?;
-            stdout.write_all(&buffer[..read - n])?;
-            for i in 0..n {
-                ring_buffer[i] = buffer[read - n + i];
-            }
+
+        total_read += read;
+
+        if total_read <= n {
+            // Fill the ring buffer without exceeding n bytes
+            let overflow = total_read - n;
+            ring_buffer.extend_from_slice(&buffer[..read - overflow]);
         } else {
-            stdout.write_all(&ring_buffer[..read])?;
-            for i in 0..n - read {
-                ring_buffer[i] = ring_buffer[read + i];
-            }
-            ring_buffer[n - read..].copy_from_slice(&buffer[..read]);
+            // Write the ring buffer and the part of the buffer that exceeds n
+            stdout.write_all(&ring_buffer)?;
+            stdout.write_all(&buffer[..read - n + ring_buffer.len()])?;
+            ring_buffer.clear();
+            ring_buffer.extend_from_slice(&buffer[read - n + ring_buffer.len()..read]);
         }
     }
+
+    Ok(())
 }
 
 fn read_but_last_n_lines(