gdb: add lookup_cmd_exact to simplify a common pattern

In code dealing with commands, there's a pattern repeated a few times of
calling lookup_cmd with some speficic arguments and then using strcmp
on the returned command to check for an exact match.
As a later patch would add a few more similar lines of code, this patch
adds a new lookup_cmd_exact function which simplify this use case.

gdb/ChangeLog:

	* cli/cli-decode.c (lookup_cmd_exact): Add.
	* cli/cli-script.c (do_define_command): Use lookup_cmd_exact.
	(define_prefix_command): Ditto.
	* command.h: Add lookup_cmd_exact.

gdb/cli/cli-decode.c

@@ -1875,6 +1875,21 @@ lookup_cmd (const char **line, struct cmd_list_element *list,
   return 0;
 }
 
+/* See command.h.  */
+
+struct cmd_list_element *
+lookup_cmd_exact (const char *name,
+		  struct cmd_list_element *list,
+		  bool ignore_help_classes)
+{
+  const char *tem = name;
+  struct cmd_list_element *cmd = lookup_cmd (&tem, list, "", NULL, -1,
+					     ignore_help_classes);
+  if (cmd != nullptr && strcmp (name, cmd->name) != 0)
+    cmd = nullptr;
+  return cmd;
+}
+
 /* We are here presumably because an alias or command in TEXT is
    deprecated and a warning message should be generated.  This
    function decodes TEXT and potentially generates a warning message

gdb/cli/cli-script.c

@@ -1391,7 +1391,7 @@ do_define_command (const char *comname, int from_tty,
       CMD_POST_HOOK
     };
   struct cmd_list_element *c, *newc, *hookc = 0, **list;
-  const char *tem, *comfull;
+  const char *comfull;
   int  hook_type      = CMD_NO_HOOK;
   int  hook_name_size = 0;
    
@@ -1403,11 +1403,7 @@ do_define_command (const char *comname, int from_tty,
   comfull = comname;
   list = validate_comname (&comname);
 
-  /* Look it up, and verify that we got an exact match.  */
+  c = lookup_cmd_exact (comname, *list);
-  tem = comname;
-  c = lookup_cmd (&tem, *list, "", NULL, -1, 1);
-  if (c && strcmp (comname, c->name) != 0)
-    c = 0;
 
   if (c && commands == nullptr)
     {
@@ -1448,11 +1444,9 @@ do_define_command (const char *comname, int from_tty,
 
   if (hook_type != CMD_NO_HOOK)
     {
-      /* Look up cmd it hooks, and verify that we got an exact match.  */
+      /* Look up cmd it hooks.  */
-      tem = comname + hook_name_size;
+      hookc = lookup_cmd_exact (comname + hook_name_size, *list,
-      hookc = lookup_cmd (&tem, *list, "", NULL, -1, 0);
+				/* ignore_help_classes = */ false);
-      if (hookc && strcmp (comname + hook_name_size, hookc->name) != 0)
-	hookc = 0;
       if (!hookc && commands == nullptr)
 	{
 	  warning (_("Your new `%s' command does not "
@@ -1593,17 +1587,12 @@ static void
 define_prefix_command (const char *comname, int from_tty)
 {
   struct cmd_list_element *c, **list;
-  const char *tem;
   const char *comfull;
 
   comfull = comname;
   list = validate_comname (&comname);
 
-  /* Look it up, and verify that we got an exact match.  */
+  c = lookup_cmd_exact (comname, *list);
-  tem = comname;
-  c = lookup_cmd (&tem, *list, "", NULL, -1, 1);
-  if (c != nullptr && strcmp (comname, c->name) != 0)
-    c = nullptr;
 
   if (c != nullptr && c->theclass != class_user)
     error (_("Command \"%s\" is built-in."), comfull);

gdb/command.h

@@ -326,6 +326,25 @@ extern struct cmd_list_element *lookup_cmd_1
 	 struct cmd_list_element **result_list, std::string *default_args,
 	 int ignore_help_classes, bool lookup_for_completion_p = false);
 
+/* Look up the command called NAME in the command list LIST.
+
+   Unlike LOOKUP_CMD, partial matches are ignored and only exact matches
+   on NAME are considered.
+
+   LIST is a chain of struct cmd_list_element's.
+
+   If IGNORE_HELP_CLASSES is true (the default), ignore any command list
+   elements which are actually help classes rather than commands (i.e.
+   the function field of the struct cmd_list_element is null).
+
+   If found, return the struct cmd_list_element for that command,
+   otherwise return NULLPTR.  */
+
+extern struct cmd_list_element *lookup_cmd_exact
+			(const char *name,
+			 struct cmd_list_element *list,
+			 bool ignore_help_classes = true);
+
 extern struct cmd_list_element *deprecate_cmd (struct cmd_list_element *,
 					       const char * );