submodule--helper update-clone: learn --init

Teach "git submodule--helper update-clone" the --init flag and remove
the corresponding shell code.

When the `--init` flag is passed to the subcommand, we do not spawn a
new subprocess and call `submodule--helper init` on the submodule paths,
because the Git machinery is not able to pick up the configuration
changes introduced by that init call. So we instead run the
`init_submodule_cb()` callback over each submodule in the same process.

[1] https://lore.kernel.org/git/CAP8UFD0NCQ5w_3GtT_xHr35i7h8BuLX4UcHNY6VHPGREmDVObA@mail.gmail.com/

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -361,14 +361,11 @@ cmd_update()
 		usage
 	fi
 
-	if test -n "$init"
-	then
-		cmd_init "--" "$@" || return
-	fi
-
 	{
-	git submodule--helper update-clone ${GIT_QUIET:+--quiet} \
+	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper update-clone \
+		${GIT_QUIET:+--quiet} \
 		${progress:+"--progress"} \
+		${init:+--init} \
 		${wt_prefix:+--prefix "$wt_prefix"} \
 		${prefix:+--recursive-prefix "$prefix"} \
 		${update:+--update "$update"} \