help: drop usage of 'common' and 'useful' for guides

Since 1b81d8cb19 (help: use command-list.txt for the source of guides,
2018-05-20), all man5/man7 guides listed in command-list.txt appear in
the output of 'git help -g'.

However, 'git help -g' still prefixes this list with "The common Git
guides are:", which makes one wonder if there are others!

In the same spirit, the man page for 'git help' describes the '--guides'
option as listing 'useful' guides, which is not false per se but can
also be taken to mean that there are other guides that exist but are not
useful.

Instead of 'common' and 'useful', use 'Git concept guides' in both
places. To keep the code in line with this change, rename
help.c::list_common_guides_help to list_guides_help.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/help.c

@@ -579,7 +579,7 @@ int cmd_help(int argc, const char **argv, const char *prefix)
 	}
 
 	if (show_guides)
-		list_common_guides_help();
+		list_guides_help();
 
 	if (show_all || show_guides) {
 		printf("%s\n", _(git_more_info_string));