fix: actually use the size hint in util_windows.go

This code was broken, it computed the size hint to do nothing with it.

util_windows.go

@@ -12,7 +12,6 @@ package flatfs
 
 import (
 	"bytes"
-	"io"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -83,19 +82,20 @@ func readFileOnce(filename string) ([]byte, error) {
 	defer f.Close()
 	// It's a good but not certain bet that FileInfo will tell us exactly how much to
 	// read, so let's try it but be prepared for the answer to be wrong.
-	var n int64 = bytes.MinRead
+	var n int = bytes.MinRead
 
 	if fi, err := f.Stat(); err == nil {
-		// As initial capacity for readAll, use Size + a little extra in case Size
-		// is zero, and to avoid another allocation after Read has filled the
-		// buffer. The readAll call will read into its allocated internal buffer
-		// cheaply. If the size was wrong, we'll either waste some space off the end
-		// or reallocate as needed, but in the overwhelmingly common case we'll get
-		// it just right.
-		if size := fi.Size() + bytes.MinRead; size > n {
-			n = size
+		if sz := int(fi.Size()); sz > n {
+			n = sz
 		}
 	}
 
-	return io.ReadAll(f)
+	var buf bytes.Buffer
+	buf.Grow(n)
+	_, err = buf.ReadFrom(f)
+	if err != nil {
+		return nil, err
+	}
+
+	return buf.Bytes(), nil
 }