Fix incorrect attribute applied when using relative line numbers.

Previously, if someone have enabled relative line numbers, and have
the attribute of active line number (the line number of the line where
the point is) different than that of normal line number, they will see
something like this:

```
 1 ...
*2*... | <- this is where the point is, *2* means 2 is highlighted.
 1 ...
*2*... this is not where the point is, but the line number is still highlighted.
 3 ...
```

src/ext/line-numbers.lisp

@@ -61,9 +61,10 @@ With a positive universal argument, use relative line numbers. Also obey the glo
             (line (line-number-at-point point))
             (custom-line (variable-value 'custom-current-line :default buffer)))
         (if (= cursor-line line)
-            (or custom-line line)
-            (abs (- cursor-line line))))
-      (line-number-at-point point)))
+            (values (or custom-line line) t)
+            (values (abs (- cursor-line line)) nil)))
+      (let ((line (line-number-at-point point)))
+        (values line (= (line-number-at-point (buffer-point buffer)) line)))))
 
 (defun get-buffer-num-format (buffer)
   (let* ((last-line (lem/buffer/internal::point-linum (buffer-end-point buffer)))
@@ -72,16 +73,15 @@ With a positive universal argument, use relative line numbers. Also obey the glo
 
 (defmethod lem-core:compute-left-display-area-content ((mode line-numbers-mode) buffer point)
   (when (buffer-filename (point-buffer point))
-    (let* ((computed-line (compute-line buffer point))
-           (num-format (or (variable-value 'line-number-format :default buffer)
-                           (get-buffer-num-format buffer)))
-           (string (format nil num-format computed-line))
-           (attribute (if (eq computed-line
-                              (compute-line buffer (buffer-point buffer)))
-                          `((0 ,(length string) active-line-number-attribute))
-                          `((0 ,(length string) line-numbers-attribute)))))
-      (lem/buffer/line:make-content :string string
-                                    :attributes attribute))))
+    (multiple-value-bind (computed-line active-line-p) (compute-line buffer point)
+      (let* ((num-format (or (variable-value 'line-number-format :default buffer)
+                             (get-buffer-num-format buffer)))
+             (string (format nil num-format computed-line))
+             (attribute (if active-line-p
+                            `((0 ,(length string) active-line-number-attribute))
+                            `((0 ,(length string) line-numbers-attribute)))))
+        (lem/buffer/line:make-content :string string
+                                      :attributes attribute)))))
 
 (defmethod lem-core:compute-wrap-left-area-content (left-side-width left-side-characters)
   (if (< 0 left-side-width)