git.c: show usage for accessing the git(1) help page

It is not immediately obvious how to use the `git help` system to show
the git(1) page, with its overview and its background and coordinating
material, such as environment variables.

Let's simply list it as the last few words of the last usage line.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -33,7 +33,8 @@ const char git_usage_string[] =
 const char git_more_info_string[] =
 	N_("'git help -a' and 'git help -g' list available subcommands and some\n"
 	   "concept guides. See 'git help <command>' or 'git help <concept>'\n"
-	   "to read about a specific subcommand or concept.");
+	   "to read about a specific subcommand or concept.\n"
+	   "See 'git help git' for an overview of the system.");
 
 static int use_pager = -1;