Print info about "git help COMMAND" on git's main usage pages

Git's main usage pages did not show "git help" as a way to get more
information on a specific subcommand. This patch adds an info line after
the list of git commands currently printed by "git", "git help", "git
--help" and "git help --all".
Signed-off-by: NTeemu Likonen <tlikonen@iki.fi>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin.h

@@ -5,6 +5,7 @@
 
 extern const char git_version_string[];
 extern const char git_usage_string[];
+extern const char git_more_info_string[];
 
 extern void list_common_cmds_help(void);
 extern void help_unknown_cmd(const char *cmd);

git.c

@@ -6,6 +6,9 @@
 const char git_usage_string[] =
 	"git [--version] [--exec-path[=GIT_EXEC_PATH]] [-p|--paginate|--no-pager] [--bare] [--git-dir=GIT_DIR] [--work-tree=GIT_WORK_TREE] [--help] COMMAND [ARGS]";
 
+const char git_more_info_string[] =
+	"See 'git help COMMAND' for more information on a specific command.";
+
 static int handle_options(const char*** argv, int* argc, int* envchanged)
 {
 	int handled = 0;
@@ -427,6 +430,7 @@ int main(int argc, const char **argv)
 		/* The user didn't specify a command; give them help */
 		printf("usage: %s\n\n", git_usage_string);
 		list_common_cmds_help();
+		printf("\n%s\n", git_more_info_string);
 		exit(1);
 	}
 	cmd = argv[0];

help.c

@@ -649,12 +649,14 @@ int cmd_help(int argc, const char **argv, const char *prefix)
 	if (show_all) {
 		printf("usage: %s\n\n", git_usage_string);
 		list_commands();
+		printf("%s\n", git_more_info_string);
 		return 0;
 	}
 
 	if (!argv[0]) {
 		printf("usage: %s\n\n", git_usage_string);
 		list_common_cmds_help();
+		printf("\n%s\n", git_more_info_string);
 		return 0;
 	}