Introduce 'submodule.recurse' option for worktree manipulators

Any command that understands '--recurse-submodules' can have its
default changed to true, by setting the new 'submodule.recurse'
option.

This patch includes read-tree/checkout/reset for working tree
manipulating commands. Later patches will cover other commands.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -855,7 +855,7 @@ static int git_checkout_config(const char *var, const char *value, void *cb)
 	}
 
 	if (starts_with(var, "submodule."))
-		return parse_submodule_config_option(var, value);
+		return submodule_config(var, value, NULL);
 
 	return git_xmerge_config(var, value, NULL);
 }