patch 8.2.2303: Vim9: backtick expansion doesn't work for :foldopen

Problem:    Vim9: backtick expansion doesn't work for :foldopen.
Solution:   Do recognize backtick expansion. (closes #7621)

Author: brammool

src/testdir/test_vim9_cmd.vim

@@ -81,6 +81,23 @@ def Test_global_backtick_expansion()
   bwipe!
 enddef
 
+def Test_folddo_backtick_expansion()
+  new
+  var name = 'xxx'
+  folddoopen edit `=name`
+  assert_equal('xxx', bufname())
+  bwipe!
+
+  new
+  setline(1, ['one', 'two'])
+  set nomodified
+  :1,2fold
+  foldclose
+  folddoclose edit `=name`
+  assert_equal('xxx', bufname())
+  bwipe!
+enddef
+
 def Test_hardcopy_wildcards()
   CheckUnix
   CheckFeature postscript

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2303,
 /**/
     2302,
 /**/

src/vim9compile.c

@@ -7505,6 +7505,13 @@ compile_exec(char_u *line, exarg_T *eap, cctx_T *cctx)
 	}
     }
 
+    if (eap->cmdidx == CMD_folddoopen || eap->cmdidx == CMD_folddoclosed)
+    {
+	// TODO: should only expand when appropriate for the command
+	eap->arg = skiptowhite(eap->arg);
+	has_expr = TRUE;
+    }
+
     if (has_expr && (p = (char_u *)strstr((char *)eap->arg, "`=")) != NULL)
     {
 	int	count = 0;