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

In 1b4735d9 (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 (6cb5728c, 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: NHan-Wen Nienhuys <hanwen@google.com>
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio 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) ||

t/t7406-submodule-update.sh

@@ -424,6 +424,16 @@ test_expect_success 'submodule update - command in .git/config catches failure -
 	test_i18ncmp actual expect
 '
 
+test_expect_success 'submodule update - command run for initial population of submodule' '
+	cat <<-\ EOF >expect
+	Execution of '\''false $submodulesha1'\'' failed in submodule path '\''submodule'\''
+	EOF &&
+	rm -rf super/submodule &&
+	test_must_fail git -C super submodule update >../actual &&
+	test_cmp expect actual &&
+	git -C super submodule update --checkout
+'
+
 cat << EOF >expect
 Execution of 'false $submodulesha1' failed in submodule path '../super/submodule'
 Failed to recurse into submodule path '../super'
@@ -476,6 +486,7 @@ test_expect_success 'submodule init picks up merge' '
 '
 
 test_expect_success 'submodule update --merge  - ignores --merge  for new submodules' '
+	test_config -C super submodule.submodule.update checkout &&
 	(cd super &&
 	 rm -rf submodule &&
 	 git submodule update submodule &&
@@ -488,6 +499,7 @@ test_expect_success 'submodule update --merge  - ignores --merge  for new submod
 '
 
 test_expect_success 'submodule update --rebase - ignores --rebase for new submodules' '
+	test_config -C super submodule.submodule.update checkout &&
 	(cd super &&
 	 rm -rf submodule &&
 	 git submodule update submodule &&