patch 8.2.2341: expresison command line completion incomplete after "g:"

Problem:    Expresison command line completion shows variables but not
            functions after "g:". (Gary Johnson)
Solution:   Prefix "g:" when needed to a global function.
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 2a6db7d..cb16de1 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -619,6 +619,10 @@
   call assert_match('"func Test_cmdline_complete_user', @:)
   call feedkeys(":func s:ScriptL\<Tab>\<Home>\"\<cr>", 'tx')
   call assert_match('"func <SNR>\d\+_ScriptLocalFunction', @:)
+
+  " g: prefix also works
+  call feedkeys(":echo g:Test_cmdline_complete_user_f\<Tab>\<Home>\"\<cr>", 'tx')
+  call assert_match('"echo g:Test_cmdline_complete_user_func', @:)
 endfunc
 
 func Test_cmdline_complete_user_names()