vim-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 vim/vim#5425)
b9adef79ec

src/nvim/eval/userfunc.c

@@ -2442,9 +2442,12 @@ void ex_function(exarg_T *eap)
         fp = NULL;
         overwrite = true;
       } else {
-        // redefine existing function
+        char_u *exp_name = fp->uf_name_exp;
+        // redefine existing function, keep the expanded name
         XFREE_CLEAR(name);
+        fp->uf_name_exp = NULL;
         func_clear_items(fp);
+        fp->uf_name_exp = exp_name;
         fp->uf_profiling = false;
         fp->uf_prof_initialized = false;
       }

src/nvim/testdir/test_vimscript.vim

@@ -1573,6 +1573,23 @@ func Test_script_local_func()
   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