Merge branch 'sb/submodule-move-head-error-msg'

"git checkout --recurse-submodules another-branch" did not report
in which submodule it failed to update the working tree, which
resulted in an unhelpful error message.

* sb/submodule-move-head-error-msg:
  submodule.c: report the submodule that an error occurs in

t/lib-submodule-update.sh

@@ -782,7 +782,8 @@ test_submodule_recursing_with_args_common() {
 		(
 			cd submodule_update &&
 			git branch -t invalid_sub1 origin/invalid_sub1 &&
-			test_must_fail $command invalid_sub1 &&
+			test_must_fail $command invalid_sub1 2>err &&
+			test_i18ngrep sub1 err &&
 			test_superproject_content origin/add_sub1 &&
 			test_submodule_content sub1 origin/add_sub1
 		)