submodule.c: report the submodule that an error occurs in

When an error occurs in updating the working tree of a submodule in
submodule_move_head, tell the user which submodule the error occurred in.

The call to read-tree contains a super-prefix, such that the read-tree
will correctly report any path related issues, but some error messages
do not contain a path, for example:

  ~/gerrit$ git checkout --recurse-submodules origin/master
  ~/gerrit$ fatal: failed to unpack tree object 07672f31880ba80300b38492df9d0acfcd6ee00a

Give the hint which submodule has a problem.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1663,7 +1663,7 @@ int submodule_move_head(const char *path,
 	argv_array_push(&cp.args, new ? new : EMPTY_TREE_SHA1_HEX);
 
 	if (run_command(&cp)) {
-		ret = -1;
+		ret = error(_("Submodule '%s' could not be updated."), path);
 		goto out;
 	}
 

t/lib-submodule-update.sh

@@ -781,7 +781,8 @@ 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
 		)