patch 8.2.3532: the previous '' mark is restored after moving the cursor

Problem:    The previous '' mark is restored after moving the cursor to the
            original jump position. (Tony Chen)
Solution:   Forget the previous position after checking. (closes #8985)
diff --git a/src/testdir/test_marks.vim b/src/testdir/test_marks.vim
index dcf4904..12501a3 100644
--- a/src/testdir/test_marks.vim
+++ b/src/testdir/test_marks.vim
@@ -26,6 +26,16 @@
   enew!
 endfunc
 
+func Test_previous_jump_mark()
+  new
+  call setline(1, ['']->repeat(6))
+  normal Ggg
+  call assert_equal(6, getpos("''")[1])
+  normal jjjjj
+  call assert_equal(6, getpos("''")[1])
+  bwipe!
+endfunc
+
 func Test_setpos()
   new Xone
   let onebuf = bufnr('%')