submodule update: run custom update script for initial populating as well

In 1b4735d9f3 (submodule: no [--merge|--rebase] when newly cloned,
2011-02-17), all actions were defaulted to checkout for populating
a submodule initially, because merging or rebasing makes no sense
in that situation.

Other commands however do make sense, such as the custom command
that was added later (6cb5728c43, submodule update: allow custom
command to update submodule working tree, 2013-07-03).

I am unsure about the "none" command, as I can see an initial
checkout there as a useful thing. On the other hand going strictly
by our own documentation, we should do nothing in case of "none"
as well, because the user asked for it.

Reported-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -605,7 +605,10 @@ cmd_update()
 		if test $just_cloned -eq 1
 		then
 			subsha1=
-			update_module=checkout
+			case "$update_module" in
+			merge | rebase | none)
+				update_module=checkout ;;
+			esac
 		else
 			subsha1=$(sanitize_submodule_env; cd "$sm_path" &&
 				git rev-parse --verify HEAD) ||