patch 8.2.3363: when :edit reuses the current buffer the alternate file is set

Problem:    When :edit reuses the current buffer the alternate file is set to
            the same buffer.
Solution:   Only set the alternate file when not reusing the buffer.
            (closes #8783)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 00d021f..434f477 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -1268,7 +1268,7 @@
 def Test_getbufline()
   e SomeFile
   var buf = bufnr()
-  e #
+  sp Otherfile
   var lines = ['aaa', 'bbb', 'ccc']
   setbufline(buf, 1, lines)
   getbufline('#', 1, '$')->assert_equal(lines)