submodule--helper: accept '-i' shorthand for update --init

commit 3ad0ba722744 ("git-submodule.sh: improve variables readability")
made `git submodules update -i` pass `-i` as is to submodule--helper,
but it fails with `error: unknown switch `i'` because the helper does
not accept the short option.

All other short options supported by git-submodule.sh are properly
handle in the helper, so also add the alias for --init

Fixes: 3ad0ba722744 ("git-submodule.sh: improve variables readability")
Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 1cc82a1..3ec8bf5 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -2990,7 +2990,7 @@ static int module_update(int argc, const char **argv, const char *prefix,
 	struct option module_update_options[] = {
 		OPT__SUPER_PREFIX(&opt.super_prefix),
 		OPT__FORCE(&opt.force, N_("force checkout updates"), 0),
-		OPT_BOOL(0, "init", &opt.init,
+		OPT_BOOL('i', "init", &opt.init,
 			 N_("initialize uninitialized submodules before update")),
 		OPT_BOOL(0, "remote", &opt.remote,
 			 N_("use SHA-1 of submodule's remote tracking branch")),