Restore info_command and breakpoint

As discussed on gdb-patches, this restores info_command and the
breakpoint on info_command in gdb-gdb.gdb.  This reverts a tiny part
of 0743fc83c0 ("Replace most calls to help_list and cmd_show_list"),
as well as 652fc23a30 ("Remove gdb-gdb.gdb breakpoint on disappeared
function info_command.").

gdb/ChangeLog
2020-05-11  Tom Tromey  <tromey@adacore.com>

	* cli/cli-cmds.c (info_command): Restore.
	(_initialize_cli_cmds): Use add_prefix_command for "info".
	* gdb-gdb.gdb.in: Restore breakpoint on info_command.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2020-05-11  Tom Tromey  <tromey@adacore.com>
+
+	* cli/cli-cmds.c (info_command): Restore.
+	(_initialize_cli_cmds): Use add_prefix_command for "info".
+	* gdb-gdb.gdb.in: Restore breakpoint on info_command.
+
 2020-05-11  Tom Tromey  <tromey@adacore.com>
 
 	* ada-lang.c (ada_value_primitive_field): Now public.

gdb/cli/cli-cmds.c

@@ -191,6 +191,17 @@ error_no_arg (const char *why)
   error (_("Argument required (%s)."), why);
 }
 
+/* This implements the "info" prefix command.  Normally such commands
+   are automatically handled by add_basic_prefix_cmd, but in this case
+   a separate command is used so that it can be hooked into by
+   gdb-gdb.gdb.  */
+
+static void
+info_command (const char *arg, int from_tty)
+{
+  help_list (infolist, "info ", all_commands, gdb_stdout);
+}
+
 /* See cli/cli-cmds.h.  */
 
 void
@@ -2189,9 +2200,9 @@ Without an argument, history expansion is enabled."),
 			   show_history_expansion_p,
 			   &sethistlist, &showhistlist);
 
-  add_basic_prefix_cmd ("info", class_info, _("\
+  add_prefix_cmd ("info", class_info, info_command, _("\
 Generic command for showing things about the program being debugged."),
-			&infolist, "info ", 0, &cmdlist);
+		  &infolist, "info ", 0, &cmdlist);
   add_com_alias ("i", "info", class_info, 1);
   add_com_alias ("inf", "info", class_info, 1);
 

gdb/gdb-gdb.gdb.in

@@ -7,6 +7,15 @@ if !$gdb_init_done
 
   b internal_error
 
+  # This provides an easy way to break into the top-level GDB by
+  # typing "info".
+  b info_command
+  commands
+    silent
+    # This avoids the voluminous output of "info".
+    return
+  end
+
   dir @srcdir@/../libiberty
   dir @srcdir@/../bfd
   dir @srcdir@