builtin: also setup gently for --help-all

Git experts often check the help summary of a command to make sure they
spell options right when suggesting advice to colleagues. Further, they
might check hidden options when responding to queries about deprecated
options like git-rebase(1)'s "preserve merges" option. But some commands
don't support "--help-all" outside of a git directory. Running (for
example)

    git rebase --help-all

outside a directory fails in "setup_git_directory", erroring with the
localized form of

    fatal: not a git repository (or any of the parent directories): .git

Like 99caeed05d (Let 'git <command> -h' show usage without a git dir,
2009-11-09), we want to show the "--help-all" output even without a git
dir. Make "--help-all" where we expect "-h" to mean
"setup_git_directory_gently", and interpose early in the natural place
("show_usage_with_options_if_asked").

Do the same for usage callers with show_usage_if_asked.

The exception is merge-recursive, whose help block doesn't use newer
APIs.

Best-viewed-with: --ignore-space-change
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

usage.c

@@ -192,7 +192,8 @@ static void show_usage_if_asked_helper(const char *err, ...)
 
 void show_usage_if_asked(int ac, const char **av, const char *err)
 {
-	if (ac == 2 && !strcmp(av[1], "-h"))
+	if (ac == 2 && (!strcmp(av[1], "-h") ||
+			!strcmp(av[1], "--help-all")))
 		show_usage_if_asked_helper(err);
 }