patch 8.2.0078: expanding <sfile> works differently the second time

Problem:    Expanding <sfile> works differently the second time.
Solution:   Keep the expanded name when redefining a function. (closes #5425)
diff --git a/src/testdir/test_vimscript.vim b/src/testdir/test_vimscript.vim
index 89112de..c890fb7 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -1642,6 +1642,23 @@
   enew! | close
 endfunc
 
+func Test_script_expand_sfile()
+  let lines =<< trim END
+    func s:snr()
+      return expand('<sfile>')
+    endfunc
+    let g:result = s:snr()
+  END
+  call writefile(lines, 'Xexpand')
+  source Xexpand
+  call assert_match('<SNR>\d\+_snr', g:result)
+  source Xexpand
+  call assert_match('<SNR>\d\+_snr', g:result)
+
+  call delete('Xexpand')
+  unlet g:result
+endfunc
+
 func Test_compound_assignment_operators()
     " Test for number
     let x = 1