vim-patch:8.2.3414: fullcommand() gives wrong name with buffer-local user command

Problem:    fullcommand() gives the wrong name if there is a buffer-local user
            command. (Naohiro Ono)
Solution:   Use a separate function to get the user command name.
            (closes vim/vim#8840)
80c88eac5a

src/nvim/ex_docmd.c

@@ -2910,7 +2910,7 @@ void f_fullcommand(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   }
 
   rettv->vval.v_string = vim_strsave(IS_USER_CMDIDX(ea.cmdidx)
-                                     ? get_user_commands(NULL, ea.useridx)
+                                     ? get_user_command_name(ea.useridx, ea.cmdidx)
                                      : cmdnames[ea.cmdidx].cmd_name);
 }
 
@@ -5155,7 +5155,7 @@ static int check_more(int message, bool forceit)
 char_u *get_command_name(expand_T *xp, int idx)
 {
   if (idx >= CMD_SIZE) {
-    return get_user_command_name(idx);
+    return expand_user_command_name(idx);
   }
   return cmdnames[idx].cmd_name;
 }
@@ -6270,7 +6270,7 @@ static void do_ucmd(exarg_T *eap)
   xfree(split_buf);
 }
 
-static char_u *get_user_command_name(int idx)
+static char_u *expand_user_command_name(int idx)
 {
   return get_user_commands(NULL, idx - CMD_SIZE);
 }
@@ -6303,6 +6303,24 @@ char_u *get_user_commands(expand_T *xp FUNC_ATTR_UNUSED, int idx)
   return NULL;
 }
 
+// Get the name of user command "idx".  "cmdidx" can be CMD_USER or
+// CMD_USER_BUF.
+// Returns NULL if the command is not found.
+static char_u *get_user_command_name(int idx, int cmdidx)
+{
+  if (cmdidx == CMD_USER && idx < ucmds.ga_len) {
+    return USER_CMD(idx)->uc_name;
+  }
+  if (cmdidx == CMD_USER_BUF) {
+    // In cmdwin, the alternative buffer should be used.
+    buf_T *buf = (cmdwin_type != 0 && get_cmdline_type() == NUL) ? prevwin->w_buffer : curbuf;
+    if (idx < buf->b_ucmds.ga_len) {
+      return USER_CMD_GA(&buf->b_ucmds, idx)->uc_name;
+    }
+  }
+  return NULL;
+}
+
 /*
  * Function given to ExpandGeneric() to obtain the list of user command
  * attributes.